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United States 
Environmental Protection 
Agency 


Manual 


Office of Research and EPA/625/R-99/010 

Development September 2000 

Cincinnati, Ohio 45268 http://www.epa.gov/ORD/NRMRL 


Constructed Wetlands 
Treatment of Municipal 
Wastewaters 
















EPA/625/R-99/010 


Manual 



Constructed Wetlands Treatment of 
Municipal Wastewaters 


National Risk Management Research Laboratory 
Office of Research and Development 
U.S. Environmental Protection Agency 
Cincinnati, Ohio 45268 



Printed on Recycled Paper 






Notice 


This document has been reviewed in accordance with the U.S. Environmental Protection 
Agency’s peer and administrative review policies and approved for publication. Mention of trade 
names or commercial products does not constitute endorsement or recommendation for use. 


, C 


1 n I 


h 




LC Control Number 



00 


329464 































Foreword 


The U.S. Environmental Protection Agency is charged by Congress with protecting the 
Nation’s land, air, and water resources. Under a mandate of national environmental laws, the 
Agency strives to formulate and implement actions leading to a compatible balance between 
human activities and the ability of natural systems to support and nurture life. To meet this 
mandate, EPA’s research program is providing data and technical support for solving environ¬ 
mental prob-lems today and building a science knowledge base necessary to manage our eco¬ 
logical re-sources wisely, understand how pollutants affect our health, and prevent or reduce 
environmen-tal risks in the future. 

The National Risk Management Research Laboratory is the Agency’s center for investiga¬ 
tion of technicological and management approaches for reducing risks from threats to human 
health and the environment. The focus of the Laboratory’s research program is on methods for 
the prevention and control of pollution to air, land, water and subsurface resources; protection 
of water quality in public water systems; remediation of contaminated sites and ground water; 
and prevention and control of indoor air pollution. The goal of this research effort is to catalyze 
development and implementation of innovative, cost-effective environmental technologies; de¬ 
velop scientific and engineering information needed by EPA to support regulatory and policy 
decisions; and provide technical support and information transfer to ensure effective implemen¬ 
tation of environmental regulations and strategies. 

This publication has been produced as part of the Laboratory’s strategic long-term research 
plan. It is published and made available by EPA’s Office of Research and Development to assist 
the user community and to link researchers with their clients. 


E. Timothy Oppelt, Director 

National Risk Management Research Laboratory 






Abstract 


This manual discusses the capabilities of constructed wetlands, a functional design ap¬ 
proach, and the management requirements to achieve the designed purpose. The manual also 
attempts to put the proper perspective on the appropriate use, design and performance of con¬ 
structed wetlands. For some applications, they are an excellent option because they are low in 
cost and in maintenance requirements, offer good performance, and provide a natural appear¬ 
ance, if not more beneficial ecological benefits. In other applications, such as large urban areas 
with large wastewater flows, they may not be at all appropriate owing to their land requirements. 
Constructed wetlands are especially well suited for wastewater treatment in small communities 
where inexpensive land is available and skilled operators hard to find and keep. 

Primary customers will be engineers who service small communities, state regulators, and 
planning professionals. Secondary users will be environmental groups and the academics. 


IV 



Contents 


Chapter 1 Introduction.1 

1.1. Scope.1 

1.2. Terminology.1 

1.3. Relationship to Previous EPA Documents.2 

1.4. Wetlands Treatment Database.2 

1.5. History.4 

1.6. Common Misperceptions.4 

1.7. When to Use Constructed Wetlands.5 

1.8 Use of This Manual.8 

1.9 References.8 

Chapter 2 Introduction to Constructed Wetlands.10 

2.1 Understanding Constructed Wetlands.10 

2.2 Ecology of Constructed Wetlands.12 

2.3 Botany of Constructed Wetlands.12 

2.4 Fauna of Constructed Wetlands.16 

2.5 Ecological Concerns for Constructed Wetland Designers.16 

2.6 Human Health Concerns.18 

2.7 Onsite System Applications.19 

2.8 Related Aquatic Treatment Systems.19 

2.9 Frequently Asked Questions.20 

2.10 Glossary.23 

2.11 References.27 

Chapter 3 Removal Mechanisms and Modeling Performance of Constructed Wetlands.30 

3.1 Introduction.30 

3.2 Mechanisms of Suspended Solids Separations and Transformations.30 

3.3 Mechanisms for Organic Matter Separations and Transformations. 35 

3.4 Mechanisms of Nitrogen Separations and Transformations.42 

3.5 Mechanisms of Phosphorus Separations and Transformations.46 

3.6 Mechanisms of Pathogen Separations and Transformations.48 

3.7 Mechanisms of Other Contaminant Separations and Transformations.49 

3.8 Constructed Wetland Modeling.50 

3.9 References.52 

Chapter 4 Free Water Surface Wetlands.55 

4.1 Performance Expectations.55 

4.2 Wetland Hydrology.64 

4.3 Wetland Hydraulics.65 

4.4 Wetland System Design and Sizing Rationale.68 

4.5 Design.69 

4.6 Design Issues.78 

4.7 Construction/Civil Engineering Issues.81 

4.8 Summary of Design Recommendations.83 

4.9 References.83 


v 


















































Contents (cont.) 


Chapter 5 Vegetated Submerged Beds.86 

5.1 Introduction.86 

5.2 Theoretical Considerations.86 

5.3 Hydrology.91 

5.4 Basis of Design.93 

5.5 Design Considerations.101 

5.6 Design Example for a VSB Treating Septic Tank or Primary Effluent.103 

5.7 On-site Applications.106 

5.8 Alternative VSB Systems.106 

5.9 References.107 

Chapter 6 Construction, Start-Up, Operation, and Maintenance.Ill 

6.1 Introduction.Ill 

6.2 Construction.Ill 

6.3 Start-Up.117 

6.4 Operation and Maintenance.118 

6.5 Monitoring.119 

6.6 References.119 

Chapter 7 Capital and Recurring Costs of Constructed Wetlands.120 

7.1 Introduction.120 

7.2 Construction Costs.120 

7.3 Operation and Maintenance Costs.125 

7.4 References.127 

Chapter 8 Case Studies.128 

8.1 Free Water Surface (FWS) Constructed Wetlands.128 

8.2 Vegetated Submerged Bed (VSB) Systems.141 

8.3 Lessons Learned.152 


VI 





























List of Figures 


2-1 Constructed wetlands in wastewater treatment train.11 

2-2 Elements of a free water surface (FWS) constructed wetland.11 

2-3 Elements of a vegetated submerged bed (VSB) system.11 

2- 4 Profile of a 3-zone FWS constructed wetland cell.18 

3- 1 Mechanisms which dominate FWS systems.32 

3-2 Weekly transect TSS concentration for Areata cell 8 pilot receiving oxidation pond effluent.34 

3-3 Variation in effluent BOD at the Areata enhancement marsh.36 

3-4 Carbon transformations in an FWS wetland.37 

3-5 Dissolved oxygen distribution in emergent and submergent zones of a tertiary FWS.40 

3-6 Nitrogen transformations in FWS wetlands.43 

3-7 Phosphorus cycling in an FWS wetland.47 

3-8 Phosphorus pulsing in pilot cells in Areata.48 

3-9 Influent versus effluent FC for the TADB systems.49 

3- 10 Adaptive model building.51 

4- 1 Effluent BOD vs areal loading.57 

4-2 Internal release of soluble BOD during treatment.57 

4-3 Annual detritus BOD load from Scirpus & Typha.58 

4-4 TSS loading vs TSS in effluent.58 

4-5 Effluent TKN vs TKN loading.59 

4-6 Effluent TP vs TP areal loading.61 

4-7 Total phosphorus loading versus effluent concentration for TADB systems.61 

4-8 Hydraulic retention time vs orthophosphate removal.62 

4-9 Influent versus effluent FC concentration for TADB systems.63 

4-10 TSS, BOD and FC removals for Areata Pilot Cell 8.63 

4-11 Tracer response curve for Sacramento Regional Wastewater Treatment Plant Demonstration 

Wetlands Project Cell 7.67 

4-12 Transect BOD data for Areata Pilot Cell 8.71 

4-13 Elements of a free water surface (FWS) constructed wetland.71 

4- 14 Generic removal of pollutants in a 3-zone FWS system.72 

5- 1 Seasonal cycle in a VSB.90 

5-2 Preferential flow in a VSB.93 

5-3 Lithium chloride tracer studies in a VSB system.94 

5-4 Effluent TSS vs areal loading rate.95 

5-5 Effluent TSS vs volumetric loading rate.95 

5-6 Effluent BOD vs areal loading rate.96 

5-7 Effluent BOD vs volumetric loading rate.96 

5-8 Effluent TKN vs areal loading rate.98 

5-9 Effluent TP vs areal loading rate.99 

5-10 NADB VSBs treating pond effluent.100 

5- 11 Proposed Zones in a VSB.102 

6- 1 Examples of constructed wetland berm construction.112 

6-2 Examples of constructed wetland inlet designs.114 


VII 














































List of Figures (cont.) 


6-3 Outlet devices.115 

8-1 Schematic diagram of wetland system at Areata, CA.129 

8-2 Schematic diagram of Phase 1 &2 wetland systems at West Jackson County, MS.132 

8-3 Schematic diagram of Phase 3 wetland expansion at West Jackson County, MS.132 

8-4 Schematic diagrams of the wetland system at Gustine, CA.136 

8-5 Schematic diagram of the wetland system at Ouray, CO.140 

8-6 Schematic of Minoa, NY, VSB system.142 

8-7 Schematic diagram of typical VSB (one of three) at Mesquite, NV.145 

8-8 Schematic of VSB system at Mandeville, LA.147 

8-9 Schematic of VSB system at Sorrento, LA.151 


VIII 













List of Tables 


1-1. Types of Wetlands in the NADB.3 

1-2. Types of Wastewater Treated and Level of Pretreatment for NADB Wetlands.3 

1-3. Size Distribution of Wetlands in the NADB.4 

1-4. Distribution of Wetlands in the NADB by State/Province.4 

1- 5. Start Date of Treatment Wetlands in the NADB.4 

2- 1 Characteristics of Plants for Constructed Wetlands.14 

2-2 Factors to Consider in Plant Selection.15 

2- 3 Characteristics of Animals Found in Constructed Wetlands.16 

3- 1 Typical Constructed Wetland Influent Wastewater.30 

3-2 Size Distributions for Solids in Municipal Wastewater.31 

3-3 Size Distribution for Organic and Phosphorus Solids in Municipal Wastewater.31 

3-4 Fractional Distribution of BOD, COD, Turbidity and TSS in the Oxidation Pond Effluent and 

Effluent from Marsh Cell 5.34 

3-5 Background Concentrations of Contaminants of Concern in FWS Wetland Treatment System Effluents.35 

3- 6 Wetland Oxygen Sources and Sinks.41 

4- 1 Loading and Performance Data for Systems Analyzed in This Document.56 

4-2 Trace Metal Concentrations and Removal Rates, Sacramento Regional Wastewater Treatment Plant.63 

4-3 Fractional Distribution of BOD, COD and TSS in the Oxidation Pond Effluent and Effluent from 

Marsh Cell 5.64 

4-4 Background Concentrations of Water Quality Constituents of Concern in FWS Constructed Wetlands.70 

4-5 Examples of Change in Wetland Volume Due to Deposition of Non-Degradable TSS (V ss ) and 

Plant Detritus (V d ) Based on 100 Percent Emergent Plant Coverage.74 

4-6 Lagoon Influent and Effluent Quality Assumptions.77 

4- 7 Recommended Design Criteria for FWS Constructed Wetlands.83 

5- 1 Hydraulic Conductivity Values Reported in the Literature.92 

5-2 Comparison of VSB Areas Required for BOD Removal Using Common Design Approaches.97 

5-3 Data from Las Animas, CO VSB Treating Pond Effluent.100 

5-4 Summary of VSB Design Guidance.106 

7-1 Cost Comparison of 4,645m 2 Free Water Surface Constructed Wetland and Vegetated Submerged Bed.121 

7-2 Technical and Cost Data for Wetland Systems Included in 1997 Case Study Visitations.121 

7-3 Clearing and Grubbing Costs for EPA Survey Sites.122 

7-4 Excavation and Earthwork Costs for EPA Survey Sites.122 

7-5 Liner Costs for EPA Survey Sites.123 

7-6 Typical Installed Liner Costs for 9,300m 2 Minimum Area.123 

7-7 Media Costs for VSBs from EPA Survey Sites.124 

7-8 Costs for Wetland Vegetation and Planting from EPA Survey Sites.124 

7-9 Costs for Inlet and Outlet Structures from EPA Sites.124 

7-10 Range of Capital Costs for a 0.4 ha Membrane-Lined VSB and FWS Wetland.126 

7-11 Annual O&M Costs at Carville, LA (570m 3 /d) Vegetated Submerged Bed.127 

7-12 Annual O&M Costs for Constructed Wetlands, Including All Treatment Costs.127 


IX 













































List of Tables (cont.) 


8-1 Summary of Results, Phase 1 Pilot Testing, Areata, CA.130 

8-2 Long-Term Average Performance, Areata WWTP.131 

8-3 Wetland Water Quality, West Jackson County, MS.134 

8-4 Performance Results in Mature Vegetated vs Immature Vegetated FWS Cells, Gustine, CA.138 

8-5 Wetland Effluent Characteristics, Gustine, CA.139 

8-6 BOD & TSS Removal for Ouray, CO.141 

8-7 Village of Minoa VSB Construction Costs.143 

8-8 Summary Performance, Mesquite, NV, VSB Component.146 

8-9 Monthly Effluent Characteristics, Mesquite, NV, VSB Component.146 

8-10 Water Quality Performance, Mandeville, LA, Treatment System (June - Sept., 1991).149 

8-11 Water Quality Performance, Mandeville, LA, Treatment System (Jan. 1996 - July 1997). 150 

8-12 VSB Effluent Water Quality, Sorrento, LA.152 


x 















Acknowledgements 


Many people participated in the creation of this manual. Technical direction throughout the 
multi-year production process was provided by USEPA’s National Risk Management Research 
Laboratory (NRMRL). Technical writing was carried out in several stages, but culminated into a 
final product as a cooperative effort between the NRMRL and the contractors named below. 
Significant technical reviews and contributions based on extensive experience with constructed 
wetlands were made by a number of prominent practitioners. Technical review was provided by 
a group of professionals with extensive experience with the problems specific to small commu¬ 
nity wastewater treatment systems. The production of the document was also a joint effort by 
NRMRL and contractual personnel. All of these people are listed below: 

Primary Authors and Oversight Committee 

Donald S. Brown, Water Supply and Water Resources Division, NRMRL, Cincinnati, OH 

James F. Kreissl, Technology Transfer and Support Division, NRMRL, Cincinnati, OH 

Robert A. Gearheart, Humboldt University, Areata, CA 

Andrew P. Kruzic, University of Texas at Arlington, Arlington, TX 

William C. Boyle, University of Wisconsin, Madison, Wl 

Richard J. Otis, Ayres Associates, Madison, Wl 

Major Contributors/Authors 

Sherwood C. Reed, Environmental Engineering Consultants, Norwich, VT 
Richard Moen, Ayres Associates, Madison, Wl 
Robert Knight, Consultant, Gainesville, FL 

Dennis George, Tennessee Technological University, Cookeville, TN 
Michael Ogden, Southwest Wetlands Group, Inc., Santa Fe, NM 
Ronald Crites, Brown and Caldwell, Sacramento, CA 
George Tchobanoglons, Consultant, Davis, CA 

Contributing Writers/Production Specialists 

Ian Clavey, CEP Inc., Cincinnati, OH 
Vince lacobucci, CEP Inc., Cincinnati, OH 
Julie Hotchkiss, CEP Inc., Cincinnati, OH 
Peggy Heimbrock, TTSD - NRMRL, Cincinnati, OH 
Stephen E. Wilson, TTSD - NRMRL, Cincinnati, OH 
Denise Ratliff, TTSD - NRMRL, Cincinnati, OH 
Betty Kampsen, STD - NRMRL, Cincinnati, OH 

Technical Reviewers 

Arthur H. Benedict, EES Consulting, Inc., Bellevue, WA 
Pio Lombardo, Lombardo Associates, Inc., Newton, MA 
Rao Surampalli, USEPA- Region VII, Kansas City, KS 

Robert K. Bastian, USEPA - Office of Wastewater Management, Washington, DC 


XI 









































































































Chapter 1 

Introduction to the Manual 


1.1. Scope 

Constructed wetlands are artificial wastewater treatment 
systems consisting of shallow (usually less than 1 m deep) 
ponds or channels which have been planted with aquatic 
plants, and which rely upon natural microbial, biological, 
physical and chemical processes to treat wastewater. They 
typically have impervious clay or synthetic liners, and en¬ 
gineered structures to control the flow direction, liquid de¬ 
tention time and water level. Depending on the type of sys¬ 
tem, they may or may not contain an inert porous media 
such as rock, gravel or sand. 

Constructed wetlands have been used to treat a variety 
of wastewaters including urban runoff, municipal, indus¬ 
trial, agricultural and acid mine drainage. However, the 
scope of this manual is limited to constructed wetlands 
that are the major unit process in a system to treat munici¬ 
pal wastewater. While some degree of pre- or post- treat¬ 
ment will be required in conjunction with the wetland to 
treat wastewater to meet stream discharge or reuse re¬ 
quirements, the wetland will be the central treatment com¬ 
ponent. 

This manual discusses the capabilities of constructed 
wetlands, a functional design approach, and the manage¬ 
ment requirements to achieve the designed purpose. This 
manual also attempts to put the proper perspective on the 
appropriate use of constructed wetlands. For some appli¬ 
cations, they are an excellent option because they are low 
in cost and in maintenance requirements, offer good per¬ 
formance, and provide a natural appearance, if not more 
beneficial ecological benefits. However, because they re¬ 
quire large land areas, 4 to 25 acres per million gallons of 
flow per day, they are not appropriate for some applica¬ 
tions. Constructed wetlands are especially well suited for 
wastewater treatment in small communities where inex¬ 
pensive land is available and skilled operators are hard to 
find. 

1.2 Terminology 

A brief discussion of terminology will help the reader dif¬ 
ferentiate between the constructed wetlands discussed in 
this manual and other types of wetlands. Wetlands are 
defined in Federal regulations as “those areas that are in¬ 
undated or saturated by surface or ground water at a fre¬ 
quency and duration sufficient to support, and that under 


normal circumstances do support, a prevalence of veg¬ 
etation typically adapted for life in saturated soil condi¬ 
tions. Wetlands generally include swamps, marshes, bogs 
and similar areas.” (40 CFR 230.3(t)) Artificial wetlands 
are wetlands that have been built or extensively modified 
by humans, as opposed to natural wetlands which are 
existing wetlands that have had little or no modification by 
humans, such as filling, draining, or altering the flow pat¬ 
terns or physical properties of the wetland. The modifica¬ 
tion or direct use of natural wetlands for wastewater treat¬ 
ment is discouraged and natural wetlands are not dis¬ 
cussed in this manual (see discussion of policy issues in 
Section 1.7.2). 

As previously defined, constructed wetlands are artifi¬ 
cial wetlands built to provide wastewater treatment. They 
are typically constructed with uniform depths and regular 
shapes near the source of the wastewater and often in 
upland areas where no wetlands have historically existed. 
Constructed wetlands are almost always regulated as 
wastewater treatment facilities and cannot be used for 
compensatory mitigation (see Section 1.7.2). Some EPA 
documents refer to constructed wetlands as constructed 
treatment wetlands to avoid any confusion about their pri¬ 
mary use as a wastewater treatment facility (USEPA, 
1999). Constructed wetlands which provide advanced 
treatment to wastewater that has been pretreated to sec¬ 
ondary levels, and also provide other benefits such as 
wildlife habitat, research laboratories, or recreational uses 
are sometimes called enhancement wetlands. 

Constructed wetlands have been classified by the lit¬ 
erature and practitioners into two types. Free water sur¬ 
face (FWS) wetlands (also known as surface flow wet¬ 
lands) closely resemble natural wetlands in appearance 
because they contain aquatic plants that are rooted in a 
soil layer on the bottom of the wetland and water flows 
through the leaves and stems of plants. Vegetated sub¬ 
merged bed (VSB) systems (also known as subsurface 
flow wetlands) do not resemble natural wetlands because 
they have no standing water. They contain a bed of media 
(such as crushed rock, small stones, gravel, sand or soil) 
which has been planted with aquatic plants. When prop¬ 
erly designed and operated, wastewater stays beneath 
the surface of the media, flows in contact with the roots 
and rhizomes of the plants, and is not visible or available 
to wildlife. 


1 




The term “vegetated submerged bed” is used in this manual 
instead of subsurface flow wetland because it is a more ac¬ 
curate and descriptive term. The term has been used previ¬ 
ously to describe these units (WPCF, 1990; USEPA, 1994). 
Some VSBs may meet the strict definition of a wetland, but a 
VSB does not support aquatic wildlife because the water 
level stays below the surface of the media, and is not condu¬ 
cive to many of the biological and chemical interactions that 
occur in the water and sediments of a wetland with an open 
water column. VSBs have historically been characterized as 
constructed wetlands in the literature, and so they are in¬ 
cluded in this manual. 

Constructed wetlands should not be confused with cre¬ 
ated or restored wetlands, which have the primary function 
of wildlife habitat. In an effort to mimic natural wetlands, the 
latter often have a combination of features such as varying 
water depths, open water and dense vegetation zones, veg¬ 
etation types ranging from submerged aquatic plants to 
shrubs and trees, nesting islands, and irregular shorelines. 
They are frequently built in or near places that have histori¬ 
cally had wetlands, and are often built as compensatory miti¬ 
gation. Created and restored wetlands for habitat or com¬ 
pensatory mitigation are not discussed in this manual. 

Finally, the term vertical flow wetland is used to describe a 
typical vertical flow sand or gravel filter which has been 
planted with aquatic plants. Because successful operation 
of this type of system depends on its operation as a filter (i.e. 
frequent dosing and draining cycles), this manual does not 
discuss this type of system. 

1.3 Relationship to Previous EPA 
Documents 

Several Offices or Programs within USEPA have published 
documents in recent years on the subject of constructed 
wetlands. Some examples of publications and their USEPA 
sponsors are: 

• Design Manual: Constructed Wetlands and Aquatic Plant 
Systems for Municipal Wastewater Treatment (1988) 
(Office of Research and Development, Cincinnati, OH, 
EPA 625-1 -88-022) 

• Subsurface Flow Constructed Wetlands for Wastewa¬ 
ter Treatment: A Technology Assessment (1993) (Office 
of Wastewater Management, Washington, DC, EPA 832- 
R-93-008) 

• Habitat Quality Assessment of Wetland Treatment Sys¬ 
tems (3 studies in 1992 and 1993) (Environmental Re¬ 
search Lab, Corvallis, OR, EPA600-R-92-229, EPA600- 
R-93-117, EPA 600-R-93-222) 

• Constructed Wetlands for Wastewater Treatment and 
Wildlife Habitat: 17 Case Studies (1993) (Office of Waste- 
water Management, Washington, DC, EPA 832-R-93- 
005) 

• Guidance for Design and Construction of a Subsur¬ 
face Flow Constructed Wetland (August 1993) (USEPA 
Region VI, Municipal Facilities Branch) 


• A Handbook of Constructed Wetlands (5 volumes, 
1995) (USEPA Region III with USDA, NRCS, ISBN 0- 
16-052999-9) 

• Constructed Wetlands for Animal Waste Treatment: A 
Manual on Performance, Design, and Operation With 
Cases Histories (1997) (USEPA Gulf of Mexico Pro¬ 
gram) 

• Free Water Surface Wetlands for Wastewater Treat¬ 
ment: A Technology Assessment (1999) (Office of 
Wastewater Management, Washington, DC, EPA/832/ 
R-99/002) 

Some information presented in this manual may contra¬ 
dict information presented in these other documents. Some 
contradictions are the result of new information and un¬ 
derstanding developed since the publication of earlier docu¬ 
ments; some contradictions are the result of earlier mis¬ 
conceptions about the mechanisms at work within con¬ 
structed wetlands; and some contradictions are the result 
of differing opinions among experts when insufficient in¬ 
formation exists to present a clear answer to issues sur¬ 
rounded by disagreement. As stated previously, this manual 
attempts to put an environmental engineering perspective 
on the use, design and performance of constructed wet¬ 
lands as reflected by the highest quality data available at 
this time. In areas where there is some disagreement 
among experts, this manual assumes a conservative ap¬ 
proach based on known treatment mechanisms which fit 
existing valid data. 

1.4 Wetlands Treatment Database 

Through a series of efforts funded by the USEPA, a 
Wetlands Treatment Database, “North American Wetlands 
for Water Quality Treatment Database or NADB” (USEPA, 

1994) has been compiled which provides information about 
natural and constructed wetlands used for wastewater 
treatment in North America. Version 1 of the NADB was 
released in 1994 and contains information for treatment 
wetlands at 174 locations in over 30 US states and Cana¬ 
dian provinces. Information includes general site informa¬ 
tion, system specific information (e.g., flow, dimensions, 
plant species), contact people with addresses and phone 
numbers, literature references, and permit information. It 
also contains some water quality data (BOD, TSS, N-se- 
ries, P, DO, and fecal conforms), but the data is not of uni¬ 
form quantity and quality, which makes it inappropriate for 
design or modeling purposes. 

Version 2 of the NADB is currently undergoing Agency 
review and contains information on treatment wetlands at 
245 locations in the US and Canada. Because each loca¬ 
tion may have multiple wetland cells, there are over 800 
individual wetland cells identified in Version 2. Besides 
expanding the number of wetland locations from Version 
1, Version 2 also contains information regarding vegeta¬ 
tion, wildlife, human use, biomonitoring and additional water 
quality data. As with Version 1, the data is not adequate 
for design or modeling. 


2 



Data did not exist or were incomplete for many of the 
wetlands included in the NADB. Only existing informa¬ 
tion was collected for the NADB; no new measurements 
were made. Therefore, the NADB is very useful for ob¬ 
taining general information about the status of con¬ 
structed wetlands usage, as well as the locations of 
operating systems and people to contact. However, it is 
not useful as a source of water quality data for wetland 
design or prediction of treatment performance. 

Tables 1.1 through 1.4 give an overview of Version 2 of 
the NADB. The size range and median size are shown in 


several tables to give the reader a feel for the size of each 
type of wetland. The median size is shown because there 
are a few very large wetlands in some of the groups, which 
makes the median size more characteristic of the group 
than the mean size. 

Tables 1.1 and 1.2 group the wetlands by type of wet¬ 
land and type of wastewater being treated, respectively. In 
general FWSs are larger than VSBs, with the median size 
of FWS wetlands being twice that of VSBs. The summary 
statistics for “other water” wetlands in Table 1.2 are some¬ 
what misleading because they are influenced by the large 
Everglades Nutrient Removal project in Florida. 


Table 1-1. Types of Wetlands in the NADB 


Type of Wetland 

Qty. 

Min. 

Size (hectares) 
Median 

Max. 

Constructed Wetlands 

205 

0.0004 

0.8 

1406 

Free Water Surface 

138 

0.0004 

1 

1406 

Marsh* 

125 

0.0004 

1 

1406 

Other 

13 

0.08 

3 

188 

Vegetated Submerged Bed (all Marsh) 

49 

0.004 

0.5 

498 

Combined FWS & VSB (all Marsh) 

8 

0.1 

0.4 

17 

Other or Not Classified 

10 

0.01 

1 

14 

Natural Wetlands (all Free Water Surface) 

38 

0.2 

40 

1093 

Forest 

18 

1 

40 

204 

Marsh 

16 

0.2 

33 

1093 

Other or Not Classified 

4 

6 

64 

494 

Not Classified 2 





'Marshes are characterized by soft-stemmed herbaceous plants, including emergent species, such as cattails, floating species, such as water lilies, 

and submerged species, such as pondweeds. (Niering, 1985) 




Table 1-2. Types of Wastewater Treated and Level of Pretreatment for NADB Wetlands 






Size (hectares) 


Wastewater Type Pretreatment 

Qty. 

Min. 

Median 

Max. 

Agricultural 

58 

0.0004 

0.1 

47 

None 

8 




Primary 

35 




Facultative 

14 




Not classified 

1 




Industrial 

13 

0.03 

10 

1093 

Primary 

1 




Facultative 

2 




Secondary 

6 




Advanced Secondary 

1 




Not classified 

3 




Municipal 

159 

0.004 

2 

500 

Primary 

9 




Facultative 

78 




Secondary 

49 




Advanced Secondary 

9 




Tertiary 

4 




Other 

4 




Not classified 

6 




Stormwater 

6 

0.2 

8 

42 

None 

4 




Secondary 

1 




Other 

1 




Other Water 

7 

3 

376 

1406 

None 

4 




Primary 

1 




Facultative 

1 




Secondary 

1 




Not classified 

2 





3 







Table 1.3 groups all the wetlands, regardless of type of 
wetland or wastewater being treated, by size. In terms of 
area, the majority of the wetlands are less than 10 hect¬ 
ares (25 acres), and almost 90% are less than 100 hect¬ 
ares (250 acres). In terms of design flow rate, the majority 
are less than 1000 m3/d (about 0.25 mgd), and 82% are 
less than 4060 m3/d (1 mgd). 

Table 1.4 groups all the wetlands, regardless of type of 
wetland or wastewater being treated, by location. Treat¬ 
ment wetlands are located in 34 US states and 6 Cana¬ 
dian provinces. The number of wetlands per state is prob¬ 
ably more a function of having an advocate for treatment 
wetlands in the state than climate or some other favorable 
condition. 


Table 1-3. Size Distribution of Wetlands in the NADB 

Area (hectares) Design Flow (m3/d) 


Size 

Range 

Cumulative 

Percentage 

Size 

Range 

Cumulative 

Percentage 

less than 1 

46 

less than 10 

19 

less than 10 

75 

less than 100 

31 

less than 100 

93 

less than 1000 

62 

less than 1000 

99 

less than 10,000 

93 


Table 1-4. Distribution of Wetlands in the NADB by State/Province 


State or Province* 

Number of 
Wetlands 

Min. 

Size (hectares) 
Median 

Max. 

SD 

42 

0.3 

2 

134 

FL 

24 

0.2 

44.5 

1406 

AR 

21 

0.3 

0.8 

4 

KY 

19 

0.01 

0.1 

5 

LA 

15 

0.02 

0.3 

17 

MS 

11 

0.02 

0.9 

101 

CA 

9 

0.1 

14 

59 

AL 

8 

0.04 

0.2 

6 

ONT 

8 

0.02 

0.09 

0.4 

MD 

5 

0.1 

0.2 

2 

OR 

5 

0.1 

4 

36 

SC 

5 

20 

36 

185 

IN 

4 

0.002 

0.12 

1 

Ml 

4 

5 

56.5 

110 

MO 

4 

0.04 

0.25 

37 

NY 

4 

0.03 

0.25 

2 

PA 

4 

0.01 

0.055 

0.2 

TX 

4 

0.1 

0.2 

0.5 

AZ 

3 

2 

38 

54 

GA 

3 

0.01 

0.3 

0.4 

ND 

3 

14 

17 

33 

NVS 

3 

0.1 

0.1 

0.4 

TN 

3 

0.1 

0.2 

0.3 

Wl 

3 

0.01 

6 

156 

ALB, IA, ME, MN, 





NC, NM, NV, QUE 

2 




CT, IL, MA, NJ, NWT, 





PEI, VA, WA 

1 





*Two-letter abbreviations are states; three-letter abbreviations are 
provinces. 


1.5 History 

Kadlec and Knight (1996) give a good historical account 
of the use of natural and constructed wetlands for waste- 
water treatment and disposal. As they point out, natural 
wetlands have probably been used for wastewater disposal 
for as long as wastewater has been collected, with docu¬ 
mented discharges dating back to 1912. Some early con¬ 
structed wetlands researchers probably began their efforts 
based on observations of the apparent treatment capacity 
of natural wetlands. Others saw wastewater as a source 
of water and nutrients for wetland restoration or creation. 
Research studies on the use of constructed wetlands for 
wastewater treatment began in Europe in the 1950's, and 
in the US in the late 1960's. Research efforts in the US 
increased throughout the 1970's and 1980's, with signifi¬ 
cant Federal involvement by the Tennessee Valley Authority 
(TVA) and the U.S. Department of Agriculture in the late 
1980's and early 1990's. USEPA has had a limited role in 
constructed wetlands research which might explain the 
dearth of useful, quality-assured data. 

Start dates for constructed wetlands in the NADB are 
shown in Table 1.5, with the start dates for natural wet¬ 
lands used for treatment included for comparison. The table 
shows that the use of FWS wetlands and VSBs in North 
America really began in the early and latel980's, respec¬ 
tively, and the number continues to increase. No new natu¬ 
ral wetland treatment systems have begun since 1990, and 
at least one-third of the natural wetland treatment systems 
included in the NADB are no longer operating. 

1.6 Common Misconceptions 

Many texts and design guidelines for constructed wet¬ 
lands, in addition to those listed above sponsored by the 
various offices of USEPA, have been published since 
USEPA’s 1988 design manual (EC/EWPCA (1990); WPCF 
(1990); Tennessee Valley Authority (1993); USDA (1993); 
Reed, et al (1995); Kadlec and Knight (1996); Campbell 
and Ogden (1999)). Also, a number of international con¬ 
ferences have been convened to present the findings of 
constructed wetlands research from almost every conti¬ 
nent (Hammer (1989); Cooper and Findlater (1990); Moshiri 


Table 1-5. Start Date of Treatment Wetlands in the NADB 


Type 

before 

1950 

1950's & 
60's 

1970's 

‘80-‘84 

‘85-‘89 

1990's 

(latest*) 

Constructed, 

FWS 

1 

0 

3 

8 

33 

85 

(‘96) 

Constructed, 

VSB 

0 

0 

0 

0 

21 

31 

(‘94) 

Constructed, 

hybrid 

0 

0 

0 

1 

4 

6 

(‘94) 

Natural, 

FWS 

4 

3 

9 

5 

8 

1 

(‘90) 


'Year of last wetland included in database for this type of wetland - other 
wetlands may have started after this date, but are not in the database. 


4 









(1993); IAWQ (1994)(1995) (1997)). However, in spite of 
the great amount of resources devoted to constructed wet¬ 
lands, questions and misconceptions remain about their ap¬ 
plication, design, and performance. This section briefly de¬ 
scribes four common misconceptions; further discussion of 
these items is found in other chapters. 

Misconception #1: Wetland design has been well-charac¬ 
terized by published design equations. Constructed wetlands 
are complex systems in terms of biology, hydraulics and water 
chemistry. Furthermore, there is a lack of quality data of suf¬ 
ficient detail, both temporally and spatially, on full-scale con¬ 
structed wetlands. Due to the lack of data, designers have 
been forced to derive design parameters by aggregating per¬ 
formance data from a variety of wetlands, which leads to 
uncertainties about the validity of the parameters. Data from 
wetlands with detailed research studies with rigorous quality 
control (QC) might be combined with data from wetlands 
with randomly collected data with little QC. Data from small 
wetlands with minimal pretreatment might be combined with 
data from large wetlands used for polishing secondary efflu¬ 
ent. Additional problems with constructed wetlands data in¬ 
clude: lack of paired influent-effluent samples; grab samples 
instead of composited samples; lack of reliable flow or de¬ 
tention time information; and lack of important incidental in¬ 
formation such as temperature and precipitation. The result¬ 
ing data combinations, completed to obtain larger data sets, 
have sometimes been used to create regression equations 
of questionable value for use in design. Finally, data from 
constructed wetlands treating relatively high quality (but in¬ 
adequately characterized) wastewater has sometimes been 
used to derive design parameters for more concentrated 
municipal treatment applications, which is less than assur¬ 
ing for any designer. 

Misconception #2: Constructed wetlands have aerobic as 
well as anaerobic treatment zones. Probably the most com¬ 
mon misconception concerns the ability of emergent wet¬ 
land plants to transfer oxygen to their roots. Emergent aquatic 
plants are uniquely suited to the anaerobic environment of 
wetlands because they can move oxygen from the atmo¬ 
sphere to their roots. Research has shown that some oxy¬ 
gen “leaks” from the roots into the surrounding soils (Brix, 
1997). This phenomenon, and early work with natural and 
constructed wetlands that treated wastewater with a low 
oxygen demand, has led to the assumption that significant 
aerobic micro-sites exist in all wetland systems. Some con¬ 
structed wetlands literature states or implies that aerobic bio¬ 
degradation is a significant treatment mechanism in fully 
vegetated systems, which has led some practitioners to be¬ 
lieve that wetlands with dense vegetation, or many sources 
of “leaking” oxygen, are in fact aerobic systems. However, 
the early work with tertiary or polishing wetlands is not di¬ 
rectly applicable to wetlands treating higher strength waste- 
water because it fails to account for the impacts of the waste- 
water on the characteristics of the wetland. Treatment mecha¬ 
nisms that function under light loads are impaired or over¬ 
whelmed due to changes imparted by the large oxygen de¬ 
mand of more contaminated municipal wastewater. Field 
experience and research have shown that the small amount 


of oxygen leaked from plant roots is insignificant compared 
to the oxygen demand of municipal wastewater applied at 
practical loading rates. 

Misconception #3: Constructed wetlands can remove sig¬ 
nificant amounts of nitrogen. Related to the misconception 
about the availability of oxygen in constructed wetlands is 
the misconception about the ability of constructed wetlands 
to remove significant amounts of nitrogen. Harvesting re¬ 
moves less than 20% of influent nitrogen (Reed, et al,1995) 
at conventional loading rates. This leaves nitrification and 
denitrification as the primary removal mechanisms. If it is 
assumed that wetlands have aerobic zones, it then follows 
that nitrification of ammonia to nitrate should occur. Further¬ 
more, if the aerobic zone surrounds only the roots of the 
plants, it then follows that anaerobic zones dominate, and 
denitrification of nitrate to nitrogen gas should also occur. 
Unfortunately, the nitrogen-related misconceptions have been 
responsible for the failure of several constructed wetlands 
that were built to remove or oxidize nitrogen. Because anaero¬ 
bic processes dominate in both VSBs and fully vegetated 
FWS wetlands, nitrification of ammonia is unlikely to occur 
in the former and will occur only if open water zones are 
introduced to the latter. Constructed wetlands can be de¬ 
signed to remove nitrogen, if sufficient aerobic (open water) 
and anaerobic (vegetated) zones are provided. Otherwise, 
constructed wetlands should be used in conjunction with other 
aerobic treatment processes that can nitrify to remove nitro¬ 
gen. 

Misconception #4: Constructed wetlands can remove sig¬ 
nificant amounts of phosphorus. Phosphorus removal in con¬ 
structed wetlands is limited to seasonal uptake by the plants, 
which is not only minor compared to the phosphorus load in 
municipal wastewater, but is negated during the plants’ se¬ 
nescence, and to sorption to influent solids which are cap¬ 
tured, soils or plant detritus, all of which have a limited ca¬ 
pacity. Two problems have been associated with phospho¬ 
rus data in the literature. First, some phosphorus removal 
data has been reported in terms of percent removal. How¬ 
ever, many of the early phosphorus studies were for natural 
wetlands or constructed wetlands that received wastewater 
with a low phosphorus concentration. Because of low influ¬ 
ent concentrations, removal of only a single mg/L of phos¬ 
phorus was reported as a large percent removal. Second, 
for studies evaluating the performance of newly constructed 
wetlands, phosphorus removal data will be uncharacteristic 
of long-term performance. New plants growing in a freshly 
planted wetland will uptake more phosphorus than a mature 
wetland, which will have phosphorus leaching from dying 
(senescent) plants as well as uptake by growing plants. Also, 
newly placed soils or media will have a greater phosphorus 
sorption capacity than a mature system which will have most 
sorption sites already saturated. 

1.7 When to Use Constructed Wetlands 

1.7.1 Appropriate Technology for Small 
Communities 

Appropriate technology is defined as a treatment sys¬ 
tem which meets the following key criteria: 


5 




Affordable - Total annual costs, including capital, op¬ 
eration, maintenance and depreciation are within the 
user’s ability to pay. 

Operable - Operation of the system is possible with 
locally available labor and support. 

Reliable - Effluent quality requirements can be con¬ 
sistently meet. 

Unfortunately, many rural areas of the U.S. with small 
treatment plants (usually defined as treating less than 3,800 
m3/d (1 mgd)) have failed to consider this appropriate tech¬ 
nology definition, and have often adopted inappropriate 
technologies such as activated sludge. In 1980, small, 
activated sludge systems constituted 39% of the small 
publicly owned treatment facilities (GAO, 1980). Recent 
information from one state showed that 73% of all treat¬ 
ment plants of less than 3,800m3/d capacity used some 
form of the activated sludge process. Unfortunately, the 
activated sludge process is considered by almost all U.S. 
and international experts to be the most difficult to operate 
and maintain of the various wastewater treatment concepts. 
Presently, small treatment plants constitute more than 90% 
of the violations of U.S. discharge standards. At least one 
U.S. state, Tennessee, has required justification for the use 
of activated sludge package plants for very small treat¬ 
ment plant applications (Tennessee Department of Public 
Health, 1977). 

Small community budgets become severely strained by 
the costs of their wastewater collection and treatment fa¬ 
cilities. Inadequate budgets and poor access to equipment, 
supplies and repair facilities preclude proper operation and 
maintenance (O&M). Unaffordable capital costs and the 
inability to reliably meet effluent quality requirements add 
up to a prime example of violating the prior criteria for ap¬ 
propriate technology. Unfortunately, no consideration for 
reuse, groundwater recharge, or other alternatives to 
stream discharge has heretofore been common, except in 
a few states where water shortages exist. 

Presently there are a limited number of appropriate tech¬ 
nologies for small communities which should be immedi¬ 
ately considered by a community and their designer. These 
include stabilization ponds or lagoons, slow sand filters, 
land treatment systems, and constructed wetlands. All of 
these technologies fit the operability criterion, and to vary¬ 
ing degrees, are affordable to build and reliable in their 
treatment performance. Because each of these technolo¬ 
gies has certain characteristics dNd requirements for pre- 
and post- treatment to meet a certain effluent quality, they 
may be used alone or in series with others depending on 
the treatment goals. 

For example, the designer may wish to supplement sta¬ 
bilization ponds with a tertiary system to meet reuse or 
discharge criteria consistently. Appropriate stabilization 
pond upgrading methods to meet effluent standards in¬ 
clude FWS wetlands, which can provide the conditions for 
enhanced settling to attain further reduction of fecal 


coliforms and removal of the excess algal growth which 
characterizes pond system effluents. FWS wetlands are 
normally used after ponds because of their ability to handle 
the excess algal solids generated in the ponds. Although 
VSBs have been employed after ponds, excess algal sol¬ 
ids have caused problems at some locations, thus defeat¬ 
ing the operability factor in the appropriate technology defi¬ 
nition. VSBs are more appropriately applied behind a pro¬ 
cess designed to minimize suspended and settleable sol¬ 
ids, such as a septic or Imhoff tank or anaerobic lagoon. 

Constructed wetlands may also require post-treatment 
processes, depending on the ultimate goals of the treat¬ 
ment system. More demanding effluent requirements may 
require additional processes in the treatment train or may 
dictate the use of other processes altogether. For example, 
the ability of constructed wetlands to remove nitrogen and 
phosphorus has frequently been overestimated. Two ap¬ 
propriate technologies that readily accomplish ammonium 
oxidation are intermittent and recirculating sand filters. 
There is at least one case study of the successful use of a 
recirculating gravel filter in conjunction with a VSB (Reed, 
et al, 1995 ). FWS systems can both nitrify and denitrify, 
thus removing significant portions of nitrogen from the 
wastewater, by alternating fully vegetated and open water 
zones in proper proportions. If the municipal facility is re¬ 
quired to have significant phosphorus removal (e.g., to at¬ 
tain 1 mg/L from a typical influent value of 6 to 7 mg/L), 
constructed wetlands will need to be accompanied by some 
process or processes that can remove the phosphorus. 

In conclusion, constructed wetlands are an appropriate 
technology for areas where inexpensive land is generally 
available and skilled labor is less available. Whether they 
can be used essentially alone or in series with other ap¬ 
propriate technologies depends on the required treatment 
goals. Additionally, they can be appropriate for onsite sys¬ 
tems where local regulators call for and allow systems other 
than conventional septic tank - soil absorption systems. 

1.7.2 Policy and Permitting Issues 

An interagency workgroup, including representatives 
from several Federal agencies, is presently developing 
Guiding Principles for Constructed Treatment Wetlands: 
Providing Water Quality and Wildlife Habitat (USEPA, 

1999). The essence of the current draft of the guidelines is 
that constructed treatment wetlands will: 

— receive no credit as mitigation wetlands; 

— be subject to the same rules as treatment lagoons 
regarding liner requirements; 

— be subject to the same monitoring requirements as 
treatment lagoons; 

— should not be constructed in the waters of the United 
States, including existing natural wetlands; and 

— will not be considered Waters of the United States 
upon abandonment if the first and the fourth condi¬ 
tions are met. 


6 




The guidance encourages use of local plant species and 
expresses concern about permit compliance during lengthy 
startup periods and vector attraction and control issues. 

To avoid additional permitting and regulatory require¬ 
ments, constructed wetlands should be designed as a treat¬ 
ment process and built in uplands as opposed to wetlands 
or flood plains, i.e., outside of waters of the U.S.. Consider 
the following from the draft guidelines. 

If your constructed treatment wetland is constructed 
in an existing water of the U.S., it will remain a water 
of the U.S. unless an individual CWA section 404 per¬ 
mit is issued which explicitly authorizes it as an ex¬ 
cluded waste treatment system designed to meet the 
requirements of the CWA.... Once constructed, if your 
treatment wetland is a water of the U.S., you will need 
a NPDES permit for the discharge of pollutants... into 
the wetland.... [Additionally,] if you wish to use a de¬ 
graded wetland for wastewater treatment and plan to 
construct water control structures, such as berms or 
levees, this construction will... require a Section 404 
permit. Subsequent maintenance may also require a 
permit. 

As stated in the guidelines: 

If the constructed wetland is abandoned or is no longer 
being used as a treatment system, it may revert to a 
water of the U.S. if... the following conditions exist: 
the system has wetland characteristics (i.e., hydrol¬ 
ogy, soils, vegetation) and it is either (1) an interstate 
wetland, (2) is adjacent to another water of the U.S. 
(other than waters which are themselves wetlands), 
or (3) if it is an isolated intrastate water which has a 
nexus to interstate commerce (e.g., it provides habitat 
for migratory birds). 

None of preceding discussion precludes designing and 
building a wetland which provides water reuse, habitat or 
public use benefits in addition to wastewater treatment. 
Constructed wetlands built primarily for treatment will gen¬ 
erally not be given credit as compensatory mitigation to 
replace wetland losses. However, in limited cases, some 
parts of a constructed wetland system may be given credit, 
especially if additional wetland area is created beyond that 
needed for treatment purposes. Also, current policy en¬ 
courages the use of properly treated wastewater to restore 
degraded wetlands. For example, restoration might be 
possible if: 

1 the source water meets all applicable water qual 
ity standards and criteria, (2) its use would result in a net 
environmental benefit to the aquatic system’s natural 
functions and values, and if applicable, (3) it would help 
restore the aquatic system to its historical condition. 
Prime candidates for restoration may include wetlands 
that were degraded or destroyed through the diversion 
of water supplies,... For example, in the arid west, there 
are often historic wetlands that no longer have a reliable 
water source due to upstream water allocations or sink¬ 


ing groundwater tables. Pre-treated effluent may be the 
only source of water available for these areas and their 
dependent ecosystems.... EPA has developed regional 
guidance to assist dischargers and regulators in dem¬ 
onstrating a net ecological benefit from maintenance of 
a wastewater discharge to a waterbody. 

This discussion of policy and permitting issues is very 
general and regulatory decisions regarding these issues 
are made on a case-by-case basis. Planners and design¬ 
ers should seek guidance from State and Regional regu¬ 
lators about site specific constructed wetland criteria in¬ 
cluding location, discharge requirements, and possible 
long-term monitoring requirements. 

1.7.3 Other Factors 

Probably the most important factor which impacts all 
aspects of constructed wetlands is their inherent aesthetic 
appeal to the general public. The desire of people to have 
such an attractive landscape enhancement treat their 
wastewater and become a valuable addition to the com¬ 
munity is a powerful argument when the need for waste- 
water treatment upgrading becomes a matter of public 
debate. The appeal of constructed wetlands makes the 
need to accurately assess the capability of the technology 
so important and so difficult. The engineering community 
often fails to appreciate this inherent appeal, while the 
environmental community often lacks the understanding 
of treatment mechanisms to appreciate the limitations of 
the technology. The natural attraction of constructed wet¬ 
lands and the potential for other aesthetic benefits may 
sometimes offset the treatment or cost advantages of other 
treatment options, and public opinion may dictate that a 
constructed wetland is the preferred option. In other situa¬ 
tions, constructed wetlands will be too costly or unable to 
produce the required effluent water quality, and the de¬ 
signer will have to convince the public that wetlands are 
not a viable option, in spite of their inherent appeal. 

The use of constructed wetlands as a treatment tech¬ 
nology carries some degree of risk for several reasons. 
First, as noted in a review of constructed wetlands for 
wastewater treatment by Cole (1998), constructed wetlands 
are not uniformly accepted by all state regulators or EPA 
regions. Some authorities encourage the use of constructed 
wetlands as a proven treatment technology, due in part to 
the misconceptions noted in Section 1.6. Others still con¬ 
sider them to be an emerging technology. As with any new 
treatment technology, uniform acceptance of constructed 
wetlands will take some time. Other natural treatment pro¬ 
cesses which are now generally accepted, such as slow 
rate or overland flow land treatment systems, went through 
a similar course of variable acceptance. 

Second, although there is no evidence of harm to wild¬ 
life using constructed wetlands, some regulators have ex¬ 
pressed concern about constructing a system which will 
treat wastewater while it attracts wildlife. Unfortunately, 
there has not been any significant research conducted on 
the risks to wildlife using constructed wetlands. Although 


7 



they are a distinctly different type of habitat, the lack of 
evidence of risks to wildlife using treatment lagoon sys¬ 
tems for many years suggests that there may not be a 
serious risk for wetlands treating municipal wastewater. 
Of course, if a wetland is going to treat wastewater with 
high concentrations of known toxic compounds, the de¬ 
signer will need to use a VSB system or incorporate fea¬ 
tures in a FWS wetland which restrict access by wildlife. 

Finally, as noted earlier, due to the lack of a large body 
of scientifically valid data, the design process is still em¬ 
pirical, that is, based upon observational data rather than 
scientific theories. Due to the variability of many factors at 
constructed wetlands being observed by researchers (e.g., 
climatic effects, influent wastewater characteristics, design 
configurations, construction techniques, and O&M prac¬ 
tices), there will continue to be disagreement about some 
design and performance issues for some period of time. 

1.8 Use of This Manual 

Chapters 1,2,7 and 8 provide information for non-tech- 
nical readers, such as decision-makers and stakeholders, 
to understand the capabilities and limitations of constructed 
wetlands. These chapters provide the type of information 
required to question designers and regulators in the pro¬ 
cess of determining how constructed wetlands may be used 
to expand, upgrade or develop wastewater treatment in¬ 
frastructure. 

Chapters 3 through 6 provide information for technical 
readers, such as design engineers, regulators and plan¬ 
ners, to plan, design, build and manage constructed wet¬ 
lands as part of a comprehensive plan for local and re¬ 
gional management of municipal wastewater collection, 
treatment, and reuse. 

Chapter 2 describes constructed wetland treatment sys¬ 
tems and their identifiable features. It answers the most 
frequently asked questions about these systems and in¬ 
cludes a glossary of terms which are used in this manual 
and generally in discussion of constructed wetland sys¬ 
tems. There are brief discussions of other aquatic treat¬ 
ment systems that are in use or are commercially avail¬ 
able and an annotated introduction to specific uses for 
constructed wetlands outside the purview of this manual. 

Chapter 3 discusses the treatment mechanisms occur¬ 
ring in a constructed wetland to help the reader under¬ 
stand the most important processes and what climatic con¬ 
ditions and other physical phenomena most affect these 
processes. A basic understanding of the mechanisms in¬ 
volved will allow the reader to more intelligently interpret 
information from other literature sources as well as infor¬ 
mation in chapters 4 and 5 of this manual. 

Chapters 4, 5, and 6 describe the design, construction, 
startup and operational issues of constructed wetlands in 
some detail. It will be apparent to the reader that there are 
presently insufficient data to create treatment models in 
which there can be great confidence. Most data in the lit¬ 


erature has been generated with inadequate quality as¬ 
surance and control (Qa/Qc), and most research studies 
have not measured or focused on documentation of key 
variables which could explain certain performance char¬ 
acteristics. Chapters 4 and 5 use the existing data of suffi¬ 
cient quality to create a viable approach to applicability 
and design of both FWS and VSB systems and sets prac¬ 
tical limits on their performance capabilities. Chapter 6 
deals with the practical issues of construction and start-up 
of these systems which have been experienced to date. 

Chapter 7 contains cost information for constructed wet¬ 
lands. Subsequent to standardizing the costs to a specific 
time, it becomes clear that local conditions and require¬ 
ments can dominate the costs. However, the chapter does 
provide a reasonable range of expected costs which can 
be used to evaluate constructed wetlands against other 
alternatives in the facility planning stage. Also, there is 
sufficient information presented to provide the user with a 
range of unit costs for certain components and to indicate 
those components that dominate system costs and those 
that are relatively inconsequential. 

Chapter 8 presents eight case studies to allow readers 
to become familiar with sites that have used constructed 
wetlands and their experiences. The systems in this chap¬ 
ter are not ones which are superior to other existing facili¬ 
ties, but they are those which have been observed and 
from which lessons can be learned by the reader about 
either successful or unsuccessful design practices. 

1.9 References 

Brix, H. 1997. Do Macrophytes Play a Role in Constructed 
Treatment Wetlands?. Water Science & Technology, 
Vol 35, No. 5, pp.11-17. 

Campbell, C.S. and M.H. Ogden. 1999. Constructed Wet¬ 
lands in the Sustainable Landscape. John Wiley and 
Sons, New York, New York. 

Cole, Stephen. 1998. The Emergence of Treatment Wet¬ 
lands. Environmental Science & Technology, Vol. 3, 
No.5, pp 218A-223A. 

Cooper, P.F., and B.C. Findlater, eds. 1990. Constructed 
Wetlands in Water Pollution Control. Pergamon Press, 
New York, New York. 

EC/EWPCA. 1990. European Design and Operations 
Guidelines for Reed Bed Treatment Systems. Prepared 
for the EC/EWPCA Expert Contact Group on Emer¬ 
gent Hydrophyte Treatment Systems. P.F. Cooper, ed., 
European Community/European Water Pollution Con¬ 
trol Association. 

Government Accounting Office. 1980. Costly wastewater 
treatment plants fail to perform as expected. CED-81- 
9. Washington, D.C. 

Hammer, D.A., ed. 1989. Constructed Wetlands for Waste- 
water Treatment. Lewis Publishers, Inc. Chelsea, 
Michigan. 


8 



IAWQ. 1992. Proceedings of international conference on 
treatment wetlands, Sydney, Australia. Water Science 
& Technology. Vol. 29, No. 4. 

IAWQ. 1995. Proceedings of international conference on 
treatment wetlands, Guangzhou, China. Water Science 
& Technology. Vol. 32, No. 3. 

IAWQ. 1997. Proceedings of international conference on 
treatment wetlands, Vienna, Austria. Water Science & 
Technology. Vol. 35, No. 5. 

Kadlec, R.H. and R.L. Knight. 1996. Treatment Wetlands. 
CRC Press LLC. Boca Raton, FL. 

Moshiri, L., ed. 1993. Constructed Wetlands for Water Qual¬ 
ity Improvement. Lewis Publishers, Inc., Chelsea, Ml. 

Niering, W.A. 1985. Wetlands. Alfred A. Knopf, Inc., New 
York, NY. 

Reed, S.C., R.W. Crites, and J.E. Middlebrooks. 1995. 
Natural Systems for Waste Management and Treat¬ 
ment. Second edition. McGraw-Hill, Inc., New York, 
NY. 

Tennessee Department of Public Health. 1977. Regula¬ 
tions for plans, submittal, and approval; Control of con¬ 
struction; Control of operation. Chapter 1200-4-2, State 
of Tennessee Administrative Rules. Knoxville, TN. 

Tennessee Valley Authority. 1993. General Design, Con¬ 
struction, and Operation Guidelines: Constructed Wet¬ 
lands Wastewater Treatment Systems for Small Us¬ 
ers Including Individual Residences. G.R. Steiner and 
J.T. Watson, eds. 2nd edition. TVA Water Management 
Resources Group. TVA/WM--93/10. Chattanooga, TN. 


U.S. Department of Agriculture. 1995. Handbook of Con¬ 
structed Wetlands. Svolumes. USDA-Natural Re¬ 
sources Conservation Service/US EPA-Region III/ 
Pennsylvania Department of Natural Resources. 
Washington, D.C. 

U.S. Environmental Protection Agency. 1988. Design 
Manual: Constructed Wetlands and Aquatic Plant Sys¬ 
tems for Municipal Wastewater Treatment. EPA/625/ 
1-88/022. US EPA Office of Research and Develop¬ 
ment, Cincinnati, OH. 

U.S. Environmental Protection Agency. 1993. Subsurface 
Flow Constructed Wetlands for Wastewater Treatment: 
A Technology Assessment. S.C. Reed, ed., EPA/ 832/ 
R-93/008. US EPA Office of Water, Washington, D.C. 

U.S. Environmental Protection Agency. 1994. Wetlands 
Treatment Database (North American Wetlands for 
Water Quality Treatment Database). R.H. Kadlec, R.L. 
Knight, S.C. Reed, and R.W. Ruble eds., EPA/600/C- 
94/002. US EPA Office of Research and Development, 
Cincinnati, OH. 

U.S. Environmental Protection Agency. 1999. Final Draft - 
Guiding Principles for Constructed Treatment Wetlands: 
Providing Water Quality and Wildlife Habitat. Developed 
by the Interagency Workgroup on Constructed Wetlands 
(U.S. Environmental Protection Agency, Army Corps of 
Engineers, Fish and Wildlife Sen/ice, Natural Resources 
Conservation Services, National Marine Fisheries Ser¬ 
vice, and Bureau of Reclamation). Final Draft 6/8/1999. 
http://www.epa.gov/owow/wetlands/constructed/ 
guide.html 

Water Pollution Control Federation. 1990. Natural Systems 
for Wastewater Treatment. Manual of Practice FD-16, 
S.C. Reed, ed., Water Pollution Control Federation, 
Alexandria, VA. 


9 




Chapter 2 

Introduction to Constructed Wetlands 


2.1 Understanding Constructed Wetlands 

Constructed wetlands are wastewater treatment systems 
composed of one or more treatment cells in a built and 
partially controlled environment designed and constructed 
to provide wastewater treatment. While constructed wet¬ 
lands have been used to treat many types of wastewater 
at various levels of treatment, the constructed wetlands 
described in this manual provide secondary treatment to 
municipal wastewater. These are treatment systems that 
receive primary effluent and treat it to secondary effluent 
standards and better, in contrast to enhancement systems 
or polishing wetlands, which receive secondary effluent 
and treat it further prior to discharge to the environment. 

This distinction emphasizes the degree of treatment more 
than the means of treatment, because the constructed 
wetlands described in this manual receive higher-strength 
wastewater than the polishing wetlands that have been 
widely used as wastewater treatment systems for the last 
20 years. 

While constructed wetlands discussed in this manual 
provide secondary treatment in a community’s wastewa¬ 
ter treatment system, this technology also can be used in 
combination with other secondary treatment technologies. 
For example, a constructed wetland could be placed up¬ 
stream in the treatment train from an infiltration system to 
optimize the cost of secondary treatment. In other uses, 
constructed wetlands could discharge secondary effluent 
to enhancement wetlands for polishing. Constructed wet¬ 
lands are not recommended for treatment of raw waste- 
water. Figure 2-1 portrays a hypothetical wastewater treat¬ 
ment train utilizing constructed wetlands in series. 

The distinction between constructed wetlands for sec¬ 
ondary treatment and enhancement systems for tertiary 
treatment is critical in understanding the limitations of ear¬ 
lier accounts of wetland-based treatment systems and 
databases of system performance. Most of the commonly 
available information on constructed wetland treatment 
systems is derived from data gathered at many larger pol¬ 
ishing wetlands and a relatively few smaller constructed 
wetlands for secondary treatment. In the past, largely un¬ 
verified data from these disparate sources has been ag¬ 
gregated, statistically rendered, and then applied as guid¬ 
ance for constructed wetland systems, with predictably 


inconsistent results. In contrast, guidance offered in this 
manual is drawn from reliable research data and practical 
application in constructed wetlands for secondary treat¬ 
ment of higher-strength municipal wastewater. 

Constructed wetlands comprise two types of systems 
that share many characteristics but are distinguished by 
the location of the hydraulic grade line. Design variations 
for both types principally affect shapes and sizes to fit site- 
specific characteristics and optimize construction, opera¬ 
tion, and performance. Both types of constructed wetlands 
typically may be fitted with liners to prevent infiltration, 
depending on local soil conditions and regulatory require¬ 
ments. 

Free water surface (FWS) constructed wetlands closely 
resemble natural wetlands in appearance and function, with 
a combination of open-water areas, emergent vegetation, 
varying water depths, and other typical wetland features. 
Figure 2-2 illustrates the main components of a FWS con¬ 
structed wetland. Atypical FWS constructed wetland con¬ 
sists of several components that may be modified among 
various applications but retain essentially the same fea¬ 
tures. These components include berms to enclose the 
treatment cells, inlet structures that regulate and distribute 
influent wastewater evenly for optimum treatment, various 
combinations of open-water areas and fully vegetated sur¬ 
face areas, and outlet structures that complement the even 
distribution provided by inlet structures and allow adjust¬ 
ment of water levels within the treatment cell. Shape, size, 
and complexity of design often are functions of site char¬ 
acteristics rather than preconceived design criteria. 

Vegetated submerged bed (VSB) wetlands consist of 
gravel beds that may be planted with wetland vegetation. 
Figure 2-3 provides a schematic drawing of a VSB sys¬ 
tem. Atypical VSB system, like the FWS systems described 
above, contains berms and inlet and outlet structures for 
regulation and distribution of wastewater flow. In addition 
to shape and size, other variable factors are choice of treat¬ 
ment media (gravel shape and size, for example) as an 
economic factor, and selection of vegetation as an optional 
feature that affects wetland aesthetics more than perfor¬ 
mance. 

The apparent simplicity and natural function of con¬ 
structed wetlands may obscure the complexity of interac- 


10 




Figure 2-1. Constructed wetlands in wastewater treatment train 


Floating and Submerged Floating and Emergent 

Inlet Settling Zone Emergent Plants Growth Plants Plants 



Zone 1 

Fully Vegetated 
D O. (-) 

H < 0.75 m 


Zone 2 

Open-Water Surface 
D O. B 
H > 1.2 m 


Zone 3 

Fully Vegetated 
D.O. B 
H < 0.75 m 


Figure 2-2. Elements of a free water surface (FWS) constructed wetland 


Pretreated 

(Settled) 

Influent 



Figure 2-3. Elements of a vegetated submerged bed (VSB) system 


11 






























































tions required for effective wastewater treatment. Unlike 
natural wetlands, constructed wetlands are designed and 
operated to meet certain performance standards. Once a 
constructed wetland is designed and becomes operational, 
the system requires regular monitoring to ensure proper 
operation. Based on monitoring results, these systems may 
need minor modifications, in addition to routine manage¬ 
ment, to maintain optimum performance. 

In this chapter, a basic understanding of constructed 
wetland ecology is presented for planners, policy makers, 
local government officials, and others involved in the ap¬ 
plication of constructed wetlands for wastewater treatment. 
Basic ecological components and functions of wetlands 
are briefly described to bring readers to a common level of 
understanding, but detailed descriptions are purposely 
omitted for the sake of focus and relative correlation to 
treatment performance. To enhance one’s knowledge of 
wetland ecology, many publications are commonly avail¬ 
able. For designers and operators, general knowledge of 
wetland ecology is assumed, and detailed information on 
constructed wetlands is offered in succeeding chapters. 
While municipal wastewater treatment systems utilizing 
constructed wetlands modeled on the functions of natural 
wetlands systems are the focus of this manual, related 
systems utilizing components of natural wetland systems 
also are briefly described. In addition, constructed wetlands 
for on-site domestic wastewater systems and non-munici¬ 
pal wastewater treatment are introduced. 

Because VSB wetlands are not dependent on wetland 
vegetation for treatment performance and do not require 
open-water areas, portions of this chapter describe de¬ 
sign and management considerations that pertain only to 
FWS wetlands. For reference purposes, important terms 
are highlighted in bold type and are explained in a glos¬ 
sary at the end of the chapter. 

2.2 Ecology of Constructed Wetlands 

Constructed wetlands are ecological systems that com¬ 
bine physical, chemical, and biological processes in an 
engineered and managed system. Successful construc¬ 
tion and operation of an ecological system for wastewater 
treatment requires a basic knowledge and understanding 
of the components and the interrelationships that compose 
the system. 

The treatment systems of constructed wetlands are 
based on ecological systems found in natural wetlands. A 
main distinction between constructed wetlands and natu¬ 
ral wetlands is the degree of control over natural processes. 
For example, a constructed wetland operates with a rela¬ 
tively stable flow of water through the system, in contrast 
to the highly variable water balance of natural wetlands, 
mostly due to the effects of variable precipitation. As a re¬ 
sult, wetland ecology in constructed wetlands is affected 
by continuous flooding and concentrations of total sus¬ 
pended solids (TSS), biochemical oxygen demand (BOD), 
and other wastewater constituents at consistently higher 
levels than would otherwise occur in nature. 


In a constructed wetland, most of the inflow is a predict¬ 
able volume of wastewater discharged through sewers. 
Lesser volumes of precipitation and surface runoff are sub¬ 
ject to seasonal and annual variations. Losses from these 
systems can be calculated by measuring outflow and esti¬ 
mating evapotranspiration as well as by accounting for 
seepage in unlined systems. Even with predictable inflow 
rates, however, modeling the water balance of constructed 
wetlands must comprehend weekly and monthly variations 
in precipitation and runoff and the effects of these vari¬ 
ables on wetland hydraulics, especially detention time re¬ 
quired for treatment. See Chapter 3 for a more thorough 
discussion of modeling concerns. 

Temperature variations also affect the treatment perfor¬ 
mance of constructed wetlands, although not consistently 
for all wastewater constituents. Treatment performance for 
some constituents tends to decrease with colder tempera¬ 
tures, but BOD and TSS removal through flocculation, sedi¬ 
mentation, and other physical mechanisms is less affected. 
In colder months, the absence of plant cover would allow 
atmospheric reaeration and solar insolation to occur with¬ 
out the shading and surface covering that plant cover pro¬ 
vides during the growing season. Ice cover is another sea¬ 
sonal variable that affects constructed wetlands by alter¬ 
ing wetland hydraulics and restricting solar insolation, at¬ 
mospheric reaeration, and biological activity; however, the 
insulating layer provided by ice cover would slow down 
the rate and degree of cooling in the water column but 
would not affect physical processes such as settling, filtra¬ 
tion, and flocculation. Plant senescence and decay also 
decreases under ice cover, with a corresponding decrease 
in effluent BOD. 

2.3 Botany of Constructed Wetlands 

Successful performance of constructed wetlands de¬ 
pends on ecological functions that are similar to those of 
natural wetlands, which are based largely on interactions 
within plant communities. Research has confirmed that 
treatment of typical wastewater pollutants (TSS and BOD) 
in FWS constructed wetlands generally is better in cells 
with plants than in adjoining cells without plants (Bavor et 
al., 1989; Burgoon et al., 1989; Gearheart et al., 1989; 
Thut, 1989). However, the mechanisms by which plant 
populations enhance treatment performance have yet to 
be determined fully. Some authors have hypothesized a 
relationship between plant surface area and the density 
and functional performance of attached microbial popula¬ 
tions (EPA, 1988; Reed et al., 1995), but demonstrations 
of this relationship have yet to be proven. 

Plant communities in constructed wetlands undergo sig¬ 
nificant changes following initial planting. Very few con¬ 
structed wetlands maintain the species composition and 
density distributions envisioned by their designers. Many 
of these changes are foreseeable, and many have little 
apparent effect on treatment performance. Other changes, 
however, may result in poor performance and the conse¬ 
quent need for increased management. The following sec¬ 
tions summarize basic principles of plant ecology that may 
aid in understanding of constructed wetlands. 


12 



2.3.1 Wetland Microbial Ecology 

In any wetland, the ecological food web requires micro¬ 
scopic bacteria, or microbes, to function in all of its com¬ 
plex transformations of energy. In a constructed wetland, 
the food web is fueled by influent wastewater, which pro¬ 
vides energy stored in organic molecules. Microbial activ¬ 
ity is particularly important in the transformations of nitro¬ 
gen into varying biologically useful forms. In the various 
phases of the nitrogen cycle, for example, different forms 
of nitrogen are made available for plant metabolism, and 
oxygen may be either released or consumed. Phosphorus 
uptake by plants also is dependent in part on microbial 
activity, which converts insoluble forms of phosphorus into 
soluble forms that are available to plants. Microbes also 
process the organic (carbon) compounds, and release 
carbon dioxide in the aerobic areas of a constructed wet¬ 
land and a variety of gases (carbon dioxide, hydrogen sul¬ 
fide, and methane) in the anaerobic areas. Plants, plant 
litter, and sediments provide solid surfaces where micro¬ 
bial activity may be concentrated. 

Microbial activity varies seasonally in cold regions, with 
lesser activity in colder months, although the performance 
differential in warm versus cold climates is less in full-scale 
constructed wetlands than in small-scale, controlled ex¬ 
periments (Wittgren and Maehlum, 1996), apparently be¬ 
cause of the multiplicity of physical, chemical, and biologi¬ 
cal transformations taking place simultaneously over a 
larger contiguous area. 

2.3.2 Algae 

Algae are ubiquitous in wet habitats, and they inevitably 
become components of FWS systems. While algae are a 
major component in certain treatment systems (for ex¬ 
ample, lagoons), algae can affect treatment performance 
of FWS constructed wetlands significantly. As a result, the 
presence of algae must be anticipated in the design stage. 

Algae in open areas, especially in areas of submergent 
vegetation, can form a living canopy that blocks sunlight 
from penetrating the water column to that vegetation, which 
results in reduced dissolved oxygen (DO) levels. The pres¬ 
ence of open, unshaded water near the outlet of a con¬ 
structed wetland typically promotes seasonal blooms of 
phytoplanktonic algal species, which results in elevated 
concentrations of suspended solids and particulate nutri¬ 
ent forms in the effluent. 

Several floating aquatic plant species, especially duck¬ 
weed, have very high rates of primary production, which 
result in large quantities of biomass and trapped nonliving 
elements accumulating within the fully vegetated portion 
of the FWS wetland and pond systems (Table 2-1). Water 
hyacinth can also perform well in pond systems in tropical 
climates to enhance TSS and algal removal. However, both 
species block sunlight and lower DO levels by eliminating 
atmospheric re aeration at the water/air interface. 

High growth rates of these plants have led to special¬ 
ized wastewater treatment systems that use these plants 
for harvesting nutrients from wastewater. The disadvan¬ 


tages of harvesting these plants arise from their low % 
solids (typically less than 5% on a wet-weight basis) and 
the consequent need for drying prior to disposal, which 
simultaneously creates secondary odor and water-quality 
problems. For disposal, harvested duckweed, which has a 
high protein content, typically has been incorporated into 
agricultural soils as green manure, and water hyacinths 
have been partially dried and landfilled or allowed to de¬ 
compose in a controlled environment to produce methane 
as a useful by-product. However, numerous attempts to 
demonstrate beneficial and cost-effective by-product re¬ 
covery have been mostly unsuccessful under North Ameri¬ 
can social and economic conditions. 

2.3.3 Emergent Herbaceous Plants 

Emergent herbaceous wetland plants are very impor¬ 
tant structural components of wetlands. Their various ad¬ 
aptations allow competitive growth in saturated or flooded 
soils. These adaptations include one or more of the fol¬ 
lowing traits: lenticels (small openings through leaves and 
stems) that allow air to flow into the plants; aerenchymous 
tissues that allow gaseous convection throughout the length 
of the plant, which provides air to plant roots; special mor¬ 
phological growth structures, such as buttresses, knees, 
or pneumatophores, that provide additional root aeration; 
adventitious roots for absorption of gases and plant nutri¬ 
ents directly from the water column; and extra physiologi¬ 
cal tolerance to chemical by-products resulting from growth 
in anaerobic soil conditions. 

The primary role of emergent vegetation in FWS sys¬ 
tems is providing structure for enhancing flocculation, sedi¬ 
mentation, and filtration of suspended solids through ide¬ 
alized hydrodynamic conditions. Emergent wetland plant 
species also play a role in winter performance of FWS 
constructed wetlands by insulating the water surface from 
cold temperatures, trapping falling and drifting snow, and 
reducing the heat-loss effects of wind (Wittgren and 
Maehlum, 1996). 

Limited information is available to demonstrate signifi¬ 
cant or consistent effects of plant species selection on 
constructed wetland performance. For example, in two simi¬ 
lar FWS treatment cells at the Iron Bridge Wetland in 
Florida, the major difference between the cells was the 
dominant plant species. Bulrush appeared to perform 
nearly the same as cattail in treatment of BOD, TSS, total 
nitrogen (TN), and total phosphorus (TP). As research and 
application of constructed wetlands have expanded, docu¬ 
mentation of actual performance differences between 
emergent marsh plant species in constructed wetlands has 
become increasingly less valuable to constructed wetland 
designers. 

The wetland designer is strongly encouraged to seek 
information from experienced local wetland practitioners 
when selecting emergent herbaceous species to ensure 
selection of locally successful species. Table 2-2 provides 
guidelines for initial selection and establishment of plant 
species adapted to wetland environments. 


13 




Table 2.1 Characteristics of plants for constructed wetlands 


General Types 
of Plants 

General Characteristics 
and Common Examples 

Function or Importance 
to Treatment Process 

Function or Importance 
for Habitat 

Design & Operational 
Considerations 

Free-Floating 

Aquatic 

Roots or root-like structures 
suspended from floating 
leaves. Will move about with 
water currents. Will not stand 
erect out of the water. 

Common duckweed (Lemna), 
Big duckweed (Spirodela). 

Primary purposes are nutrient 
uptake and shading to retard 
algal growth. Dense floating mats 
limit oxygen diffusion from the 
atmosphere. Duckweed will be 
present as an invasive species. 

Dense floating mats limit 
oxygen diffusion from the 
atmosphere and block 
sunlight from submerged 
plants. Plants provide shelter 
and food for animals. 

Duckwood is a natural 
invasive species in 
North America. No 
specific design is 
required. 

Rooted Floating 
Aquatic 

Usually with floating leaves, 
but may have submerged 
leaves. Rooted to bottom. 

Will not stand erect out of the 
water. Water lily (Nymphea), 
Pennywort (Hydrocotyle). 

Primary purposes are providing 
structure for microbial attachment 
and releasing oxygen to the water 
column during daylight hours. 

Dense floating mats limit oxygen 
diffusion from the atmosphere. 

Dense floating mats limit 
oxygen diffusion from the 
atmosphere and block 
sunlight from submerged 
plants. Plants provide 
shelter and food for animals. 

Water depth must be 
designed to promote 
the type of plant (i.e. 
floating, submerged, 
emergent) desired 
while hindering other 
types of plants. 

Submerged 

Aquatic 

Usually totally submerged; 
may have floating leaves. 

Rooted to bottom. Will not 
stand erect in air. Pondweed 

(Potamogeton), Water weed 
(Elodea). 

Primary purposes are provioing 
structure for microbial attachment, 
and providing oxygen to the water 
column during daylight hours. 

Plants provide shelter and 
food for animals (especially 
fish). 

Retention time in open 
water zone should be 
less than necessary 
to promote algal 
growth which can 
destroy these plants 
through sunlight 
blockage. 

Emergent 

Aquatic 

Herbaceous (i.e. non-woody). 
Rooted to the bottom. Stand 
erect out of the water. Tolerate 
flooded or saturated conditions. 
Cattail (Typha), Bulrush 
(Scirpus), Common Reed 
(Phragmites). 

Primary purpose is providing 
structure to induce enhanced 
flocculation and sedimentation. 
Secondary purposes are shading 
to retard algal growth, windbreak 
to promote quiescent conditions for 
settling, and insulation during winter 
months. 

Plants provide shelter and 
food for animals. Plants 
provide aesthetic beauty for 
humans. 

Water depths must be 
in the range that is 
optimum for the 
specific species 
chosen (planted). 

Shrubs 

Woody, less than 6 m tall. 
Tolerate flooded or saturated 
soil conditions. Dogwood 
(Cornus), Holly (Ilex). 

Treatment function is not defined: 
it is not known if treatment data 
from unsaturated or occasionally 
saturated phytoremediation sites 
in upland areas is applicable to 
continuously saturated wetland 
sites. 

Plants provide shelter and 
food for animals (especially 
birds). Plants provide aesthetic 
beauty for humans. 

Possible perforation of 
liners by roots. 

Trees 

Woody, greater than 6 m tall. 

(same as for shrubs) 

(same as for shrubs) 

(same as for shrubs) 


Tolerate flooded or saturated 
soil conditions. Maple (Acer). 
Willow (Salix). 


2.3.4 Plant Nutrition and Growth Cycles 

Wetland plants require optimum environmental condi¬ 
tions in each phase of their life cycles, including germina¬ 
tion and initial plant growth, adequate nutrition, normal 
seasonal growth patterns, and rates of plant senescence 
and decay. For more detailed information on wetland plant 
ecology, the nonbiologist is referred to the wetland ecol¬ 
ogy text by Mitsch and Gosselink (1993) and portions of 
the constructed wetland text by Kadlec and Knight (1996). 

A wide variety of references describe growth cycles, tim¬ 
ing of seed release, overwintering ability, energy cycling, 
and other characteristics and processes that provide wet¬ 
land plant species with a competitive advantage in their 


natural habitats; the reader is referred to other sources for 
detailed information. An overview of important character¬ 
istics follows. 

Emergent herbaceous wetland species planted early in 
the growing season in temperate climates generally multi¬ 
ply by vegetative reproduction to a maximum total stand¬ 
ing biomass in late summer or early fall within a single 
growing season. This biomass may represent multiple 
growth and death periods for individual plants during the 
course of the growing season, or it may represent a single 
emergence of plant structures, depending on the species. 
For many species, seeds are produced along with maxi¬ 
mum standing crop and released with maturation in the 
fall for early germination in the spring. 


14 











Table 2.2 Factors to Consider in Plant Selection (adapted from Thunhorst, 1993) 


Factors 


Consult local experts 


Native species 

Invasive or aggressive 
species 

Tolerant of high 
nutrient load 


Comments 


The number of professional wetland scientists, practitioners, and plant nurseries has increased dramatically in the 
past 10 years. Help from an experienced, local person should be available from a variety of sources, including 
government agencies and private companies. 

Using plants that grow locally increases the likelihood of plant survival and acceptance by local officials. 

Plants that have extremely rapid growth, lack natural competitors, or are allelopathic* can crowd out all other spe¬ 
cies and destroy species diversity. State or local agencies may ban the use of some species. 

Unlike natural wetlands, constructed wetlands will receive a continuous inflow of wastewater with high nutrient 
concentrations. Plants that can not tolerate this condition will not survive. 


Tolerant of continuous Unlike natural wetlands, which may experience periodic or occasional dry periods, constructed wetlands will 

flooding receive a continuous inflow of wastewater. Plants that require periodic or occasional drying as part of their 

reproductive cycle will not survive. 

Growth characteristics Perennial plants are generally preferred over annual plants because plants will continue growing in the same area 

and there is no concern about seeds being washed or carried away. For emergent species, persistent plants are 
generally preferred over semi- or non-persistent plants because the standing plant material provides added 
shelter and insulation during the winter season.f 


Available form for planting Costs of obtaining and planting the plants will vary depending on the form of planting material, which may be 

available in a variety of forms depending on the plant species. Entire plant forms (e.g. bare root plants or plugs) will 
usually cost more than partial plant material (e.g. seeds or rootstock), but the plant supplier may guarantee a 
higher survival rate.T 


Rate of growth 


Slower growing plants will require a greater number of plants, planted closer together, at start-up to obtain the 
same density of plant coverage in the initial growing season. 


Wildlife benefits 


If the wetland is to be used for habitat, plants that provide food, shelter/cover and nesting/nursery for the desired 
animals should be chosen. 


Plant diversity Mono-cultures of plants are more susceptible to decimation by insect or disease infestations; catastrophic 

infestations will temporarily affect treatment performance. Greater plant diversity will also tend to encourage a 
greater diversity of animals. 


* Allelopathic - plants that have harmful effects on other plants by secreting toxic chemicals 

t Perennial - aboveground portion dies, but below-ground portion remains dormant and sprouts in the next growing season. 

Annual - entire plant dies and reproduction is only by seed produced before the plant dies. 

Persistent - aboveground dead portions remain upright through the dormant season. 

Semi-persistent - aboveground dead portions may remain standing for some part of the dormant season before falling into clumps. 
Non-persistent - aboveground dead portions decay and wash away at the end of the growing season. 
t Bare root plant - seedling with soil washed from roots. Plug - seedling with soil still on roots. Rootstock - piece of underground stem (rhizome). 


For some species with high lignin content, particularly 
cattail, bulrush, and common reed, much of the plant re¬ 
mains standing as dead biomass that slowly decays dur¬ 
ing the winter season. In FWS systems, this standing dead 
biomass provides additional structure for enhanced floc¬ 
culation and sedimentation that is important in wetland 
treatment performance throughout the annual cycle. Dead 
biomass, both standing and fallen, also is important to root 
viability under flooded, winter conditions because of the 
insulating layer it provides, in addition to its contribution to 
the internal load on the system. 

Like all plants, wetland plants require many macro- and 
micronutrients in proper proportions for healthy growth. 
While municipal wastewater can supply adequate quanti¬ 
ties of these limiting nutrients, other types of wastewater, 
including industrial wastewater, acid mine drainage, and 
stormwater, may not. 


Nitrogen and phosphorus are key nutrients in the life 
cycles of wetland plants. However, plant uptake of nitro¬ 
gen and phosphorus is not a significant mechanism for 
removal of these elements in most wetlands receiving par¬ 
tially treated municipal wastewater because nitrogen and 
phosphorus are taken up and released in the cycle of plant 
growth and death. Nonetheless, undecomposed litter from 
dead biomass provides storage for phosphorus, metals, 
and other relatively conservative elements (Kadlec and 
Knight, 1996). 

While uptake rates of nitrogen and phosphorus are po¬ 
tentially high, harvesting plant biomass to remove these 
nutrients has been limited to floating aquatic plant com¬ 
munities, in which the plants can be harvested with only 
brief altering of system performance. Although common 
reed is harvested annually from certain European con¬ 
structed wetlands as a by-product (and not for nutrient re¬ 
duction), full-scale constructed wetlands where plants are 


15 









routinely harvested have not been documented in the 
United States. 

2.4 Fauna of Constructed Wetlands 

The role that animal species may play in constructed 
wetlands is a consideration for management of FWS wet¬ 
lands. Animals typically compose less biomass than do 
wetland plants, but animals are able to alter energy and 
mass flows disproportionately to their biomass contribu¬ 
tion. During outbreaks of insect pests in constructed wet¬ 
lands, for example, entire marshes and floating aquatic 
plant systems can be defoliated, which interrupts mineral 
cycles and upsets water-quality treatment performance. 
In another example, the rooting action of bottom-feeding 
fish (primarily carp) causes sediment resuspension, which 
affects performance of constructed wetlands in removing 
suspended solids and associated pollutants. The presence 
of large seasonal waterfowl populations has had similar 
results in constructed wetlands at Columbia, Missouri, and 
elsewhere. In VSB wetlands, only avian species play a 
significant role. 

While wildlife species play generally positive, second¬ 
ary roles in constructed wetlands, their presence also may 
generate unintended consequences. Bird species common 
to wetland environments, for example, typically attract 
birdwatchers, who may provide public support for munici¬ 


palities and industries employing this treatment technol¬ 
ogy. The presence of the public at constructed wetlands 
for secondary treatment, however, necessitates manage¬ 
ment efforts to ensure adequate protection from human 
health and safety risks presented by exposure to primary 
effluent (see also section 2.6). Conversely, regulatory con¬ 
cern for potentially vulnerable wildlife species has impeded 
plans for constructed wetlands at certain sites and for cer¬ 
tain wastewaters with toxic constituents. 

Free water surface wetlands closely resemble the ecol¬ 
ogy of natural wetlands and aquatic habitats, and they in¬ 
evitably attract animal species that rely on wet environ¬ 
ments during some or all of their life history. All animal 
groups are represented in constructed wetlands: protozo¬ 
ans, insects, mollusks, fish, amphibians, reptiles, birds, and 
mammals. Table 2-3 summarizes animal species that may 
be found in constructed wetlands. 

2.5 Ecological Concerns for Constructed 
Wetland Designers 

Wetland ecology is integral to the success of constructed 
wetlands because of their complexity and their accessibil¬ 
ity to wildlife. While the ecology of VSB systems relates 
more to its subsurface than its surface environment, wet¬ 
land plants and other surface features that are character¬ 
istic of VSB wetlands also require consideration. 


Table 2.3 Characteristics of Animals Found in Constructed Wetlands 

Members of Group Commonly Found in Function or Importance to Treatment 
Animal Group Treatment Wetlands Process Design & Operational Considerations 


Invertebrates, 
including 
protozoa, 
insects, spiders, 
and crustaceans 

A wide variety will be present, but diversity 
and populations will vary seasonally and 
spatially. 

Undoubtedly play a role in 
chemical and biological cycling and 
transformations and in supporting food 
web for higher organisms, but exact 
functions have not been defined 

Mosquito control must be considered; 
mono-cultures of plants are more 
susceptible to decimation by insect 
infestations. 

Fish 

Species adapted to living at or near the 
surface (mosquitofish, mudminnow); 
species adapted to living in polluted 
waters (bowfin, catfish, killifish, carp). 

Consumers of insects and decaying 
material (e.g. mosquitofish eat 
mosquito larvae). 

Anaerobic conditions will limit 
populations; nesting areas required; 
bottom-feeders can uproot plants and 
resuspend sediments. 

Amphibians and 
Reptiles 

Frogs, alligators, snakes, turtles 

Consumers of lower organisms 

Turtles have an uncanny ability to fall 
into water control structures and to 
get caught in pipes, so turtle exclusion 
devices are needed; monitoring of 
control structures and levees for 
damage or obstruction is needed. 

Birds 

A wide variety (35-63 species') are 
present, including forest and prairie 
species as well as waterfowl, but 
diversity and populations vary 
seasonally and spatially. 

Consumers of lower organisms 

Heavy use, especially by migratory 
waterfowl, can contribute to pollutant 
load on a seasonal basis. 

Mammals 

Small rodents (shrews, mice, voles); 
large rodents (rabbits, nutria, muskrats, 
beaver); large grazers (deer); large 
carnivores (opossums, raccoons, foxes). 

Consumers of plants and lower 
organisms 

Nutria and muskrat populations can 
reach nuisance levels, removing 
vegetation and destroying levees; 
structural controls and animal 
removal may be required. 


* McAllister, 1992, 1993a, 1993b 


16 









Constructed wetlands invariably attract wildlife, a factor 
that must be considered in the design and management of 
constructed wetlands. As components of an ecological 
community, animals in general perform vital ecological func¬ 
tions in constructed wetlands. Specific roles of animals in 
the development and operation of constructed wetlands, 
however, are not well researched. Experience has shown 
that many animals are beneficial elements in constructed 
wetlands, but many other are nuisance species. Proper 
attention to desirable and undesirable wildlife species, as 
well as primary and ancillary functions of constructed wet¬ 
lands, will aid the success of a constructed wetland. 

2.5.1 Primary and Ancillary Functions of 
Constructed Wetlands 

Primary functions of most constructed wetlands include 
water storage and water-quality improvement. Some of 
these constructed wetlands are designed intentionally for 
ground water recharge. Numerous other functions attrib¬ 
uted to natural wetlands are important in constructed wet¬ 
lands and are described in succeeding chapters. 

Ancillary functions include primary production of organic 
carbon by plants; oxygen production through photosyn¬ 
thesis; production of wetland herbivores, as well as preda¬ 
tor species that range beyond the wetland boundaries; 
reduction of export of organic matter and nutrients to down¬ 
stream ecosystems; and creation of cultural values in terms 
of educational and recreational resources. One or more of 
these ancillary functions may be an important goal in some 
constructed wetland projects. For detailed descriptions of 
ancillary functions, the reader is referred to information 
presented elsewhere (Feierabend, 1989; Sather, 1989; 
Knight, 1992). 

2.5.2 Wildlife Access Controls 

Successful wildlife management in FWS wetlands re¬ 
quires maintaining a balance between attracting benefi¬ 
cial species and controlling pest species (EPA, 1993a). 
While most wildlife species in wetlands are attractive but 
often unnoticed, many species are attractive for aesthetic 
reasons but are impediments to the success of constructed 
wetlands. Nuisance species in constructed wetlands in¬ 
clude burrowing rodents, especially beavers, nutria, and 
muskrats, which burrow through berms and levees and 
consume beneficial emergent vegetation; mosquitoes, 
which cause annoyance and health concerns; and certain 
bottom-feeding fish, such as carp, which uproot aquatic 
vegetation and cause increases in effluent TSS and asso¬ 
ciated pollutants by stirring up sediments and resuspend¬ 
ing them in the water column. Waterfowl in large numbers 
also may be undesirable because they cause similar prob¬ 
lems, and their nutrient-rich droppings place additional 
demands on the water-quality performance of constructed 
wetlands. 

Control of wildlife access in constructed wetlands is highly 
site-specific; as a result, control measures must be based 
on geographic location, nuisance species, wetland design, 
and preferred levels of management. Control methods are 


applied throughout the planning, construction, and opera¬ 
tion of constructed wetland projects. Control of carp, for 
example, can be anticipated during design and managed 
with winter drawdown of water levels and subsequent in- 
depth freezing in northern climates. Also effective is draw¬ 
down and physical removal of stranded individuals, but 
this method is more labor intensive and less effective in 
eradicating carp populations. Large rodents can be 
screened out of culverts to limit access and prevent dam¬ 
ming; however, trapping and physical removal may be 
needed to prevent burrowing and subsequent undermin¬ 
ing of banks and other damage. For waterfowl control, lim¬ 
ited open-water areas will discourage many species, but 
treatment requirements will dictate the size and use of these 
zones. Netting suspended over unavoidable open-water 
areas can prevent their use for feeding, but this method 
deviates from the intent to incorporate natural methods of 
wildlife control. 

Wetland wildlife species frequently have home ranges 
well outside the borders of an individual constructed wet¬ 
land cell; consequently, they can become a public resource 
that may need to be protected and promoted for reasons 
unrelated to their perceived value to constructed wetlands. 
Although the values of constructed wetlands for wildlife 
habitat may be subject to public and scientific debate, this 
topic nonetheless must be considered in all project phases 
to determine optimum design and management features 
to promote or discourage the presence of wildlife (Knight, 
1997; Worrall et al., 1996). 

2.5.3 Mosquito Habitat Controls 

Mosquitoes may be integral components of the ecologi¬ 
cal food web, but mosquitoes generally are considered a 
pest species. While a constructed wetland’s attractiveness 
to wildlife may be regarded as a benefit to the human com¬ 
munity, the potential for breeding mosquitoes can be an 
obstacle to permitting, funding, and other steps essential 
to the siting of a constructed wetland. 

Several methods of mosquito control can be employed 
in the planning, construction, and operation of constructed 
wetlands. Predation is one means. Mosquito fish have been 
found to be effective in reducing mosquito populations when 
habitat conditions are optimized by manipulating water lev¬ 
els and when channels are kept free of dead vegetation. 
Drawdown of water levels aids mosquito fish spawning in 
spring and provides the fish with better access to mos¬ 
quito larvae during mosquito breeding season (Dill, 1989). 
In warm climates, mosquito fish habitat must be monitored 
for excessive water temperatures and fluctuations in efflu¬ 
ent strength and content. Bats and several avian species 
also are effective predators, but planning and managing 
optimum conditions have yet to be standardized. 

In the planning and construction stages, management 
of mosquito habitat can be enabled with steep slopes on 
water channels that reduce standing water area in shallow 
areas. In contrast to this design is the use of more natural, 
undulating banks that have been popular in polishing wet- 


17 




lands. This natural appearance is more visually appealing 
but is ineffective for mosquito-control purposes. A channel 
profile that has been effective in mosquito control is a steep¬ 
sided channel flanked by relatively flat aprons leading out¬ 
ward to steep-sided banks (Dill, 1989). This profile allows 
the facility operator to draw down water levels to the lower 
channel during the mosquito-breeding season. Figure 2-4 
illustrates this design. With standing water eliminated from 
emergent vegetation in the shallow flanks of the channel, 
deeper water in the lower channel provides an environ¬ 
ment more conducive to mosquito predation by fish spe¬ 
cies. Flexible drainage capability is essential to this means 
of control. 

Water spray systems also have been used for mosquito 
control, but such mechanical systems are inconsistent with 
the passive nature of constructed wetlands, which utilize 
natural systems to accomplish wastewater treatment and 
manage ancillary concerns. 

Vegetation management is another approach to mos¬ 
quito control, especially in the absence of water-level con¬ 
trol features (Dill, 1989). Taller vegetation especially needs 
management. Cattails and bulrushes, for example, tend to 
fall over late in the growing season, which creates condi¬ 
tions favorable for mosquito reproduction in the following 
growing season, as well as unfavorable conditions for pre¬ 
dation by mosquito fish (Martin and Eldridge, 1989). Chan¬ 
nels planted with lower-growing vegetation and cleared 
annually of dead standing stock can reduce mosquito popu¬ 
lations and optimize predation, providing that this vegeta¬ 
tion imparts the same structural role beneath the water 
surface. 

Larvicide is a proven means of active mosquito control 
when employed in conjunction with other management 
techniques. A bacterium (Bacillus sphaericus) has been 


found effective in reducing culex mosquitoes, one of the 
most common species in the United States. Tests have 
indicated that a commercial larvicide containing the bacte¬ 
ria may be capable of eliminating most of the populations 
of culex in treatment lagoons (WaterWorld, 1996). The 
concentrated bacteria in powdered form is applied to stand¬ 
ing water as a coating on granulated corncobs, which 
quickly releases protein crystals and bacteria spores to 
the water surface. Upon ingestion, the bacteria enter mos¬ 
quito larvae tissues through pores in the gut wall and mul¬ 
tiply rapidly, and the infected larvae typically die within two 
days. However, fully vegetated zones are more difficult to 
treat than open water zones or lagoons. 

2.6 Human Health Concerns 

Many studies of constructed wetlands’ biological effec¬ 
tiveness and attractiveness to humans for aesthetic and 
cultural reasons have focused on polishing wetlands that 
receive secondary effluent, which are outside the focus of 
this manual. At many of these successful polishing wet¬ 
lands for tertiary treatment, interpretive centers and signage 
invite visitors, and boardwalks and naturalists guide them 
through the outdoor experience. Constructed wetlands that 
receive primary effluent for secondary treatment, on the 
other hand, may not be visitor-friendly places, and human 
visitors may best enjoy them from the periphery for sev¬ 
eral reasons. 

Partially treated wastewater in a constructed wetland for 
secondary treatment, despite the proven effectiveness of 
this ecological approach to treatment, presents essentially 
the same risks to human health as wastewater in primary 
treatment and lagoons. Risk of dermal contact and pos¬ 
sible transmission of disease is equally unappealing in FWS 
wetlands for secondary treatment as it is in open lagoons. 
This concern is distinguished from human interaction with 



Figure 2-4. Profile of a three-zone FWS constructed wetland cell 


18 





















polishing systems, where influent wastewater has already 
met effluent quality requirements which are set by regula¬ 
tory authorities. 

In constructed wetlands receiving primary effluent, hu¬ 
man exposure to wastewater is a greater concern at the 
inlet end of the system, where influent has achieved pri¬ 
mary treatment only. Lesser concern for human exposure 
is warranted at the outlet end, where wastewater has been 
treated to the quality of secondary treatment or better, which 
is the quality of wastewater entering the polishing wetlands 
that have been popular for environmental awareness and 
education activities. 

As a result, humans must be considered an unwanted 
species in most areas of FWS wetlands treating municipal 
wastewater to meet secondary treatment (defined as 30 
mg/L of BOD and TSS). Nonetheless, constructed wet¬ 
lands can serve as recreational areas and outdoor labora¬ 
tories, especially at the outlet end where wastewater has 
been treated to secondary effluent standards. Management 
considerations may include the public’s access, percep¬ 
tions, and exposure to health threats (Knight, 1997). To 
effectively address these concerns, fencing, signage, and 
other controls must be considered in the proposal stage 
as well as in design and operation of the system. 

Mosquito populations may represent merely an annoy¬ 
ance factor to be managed, as described above, but some 
species of mosquitoes also carry a health risk that must 
be addressed. In warmer climates, including the southern 
United States, the encephalitis mosquito (Culex tarsalis) 
thrives in the extended breeding season provided by con¬ 
structed wetlands, but water-level manipulation and mos¬ 
quito fish predation in the two-tiered pond design described 
previously have been effective in controlling these mos¬ 
quito populations (Dill, 1989). The two-tiered design allows 
water levels to be drawn down to concentrate prey spe¬ 
cies (mosquitoes) in smaller areas for more efficient con¬ 
sumption by predators (mosquito fish). 

Most of the health concerns described above do not apply 
to VSB systems, in which wastewater typically is not ex¬ 
posed at the land surface. 

2.7 On-site System Applications 

On-site constructed wetland systems may also be ap¬ 
plied to wastewater treatment and disposal at individual 
properties. On-site constructed wetlands generally utilize 
the same technologies as the municipal VSB systems de¬ 
scribed in this manual, and they share with municipal sys¬ 
tems the advantages of cost-effectiveness and low-main¬ 
tenance requirements. However, on-site constructed wet¬ 
lands are distinguished typically by final effluent discharge 
to soils instead of surface water. For purposes of this dis¬ 
cussion, on-site constructed wetland systems treat septic 
tank effluent, or primary effluent, in small-scale VSB sys¬ 
tems for subsurface disposal to soils. 

On-site constructed wetlands also differ from municipal 
systems in scale. On-site constructed wetlands typically 


occupy only a few hundred square feet. Municipal VSB 
systems may serve hundreds of residential, commercial, 
and industrial properties, while on-site systems would serve 
a single home or several residences in a cluster. 

An on-site VSB system typically consists of a lined VSB 
that receives primary effluent from a septic tank, and in 
some designs, a second VSB that receives effluent from 
the upstream VSB system. The second VSB can be un¬ 
lined to allow treated wastewater to infiltrate to soil for dis¬ 
posal. Variations of this treatment train include use of 
supplemental absorption trenches to facilitate soil absorp¬ 
tion and direct surface discharge with or without subse¬ 
quent disinfection. Each VSB typically is planted with wet¬ 
land vegetation. 

Applied studies and research experiments of on-site 
constructed wetland systems have shown adequate treat¬ 
ment performance for most wastewater constituents, in¬ 
cluding BOD, TSS, and fecal coliforms, with variations in 
performance for removal of ammonia nitrogen (Burgan and 
Sievers, 1994; Huang et al., 1994; Johns et al., 1998; 
Mankin and Powell, 1998; Neralla et al., 1998; White and 
Shirk, 1998). 

2.8 Related Aquatic Treatment Systems 

Several types of aquatic treatment systems have been 
constructed to treat municipal and other wastewaters, and 
most of these systems fall outside the definition of con¬ 
structed wetlands discussed in this manual. These other 
types of systems are briefly described to provide the reader 
with additional background and references to source ma¬ 
terial. 

Polishing wetlands have been used also to remove trace 
metals, including cadmium, chromium, iron, lead, manga¬ 
nese, selenium, and zinc in a variety of situations. The 
primary removal mechanism for metals in wastewater ap¬ 
pears to be sedimentation. Plant uptake results in deposi¬ 
tion of metals to soil via plant roots and requires harvest of 
plants to partially remove metals from the system. In some 
cases, however, effluent concentrations of metals have 
exceeded influent levels, apparently due to evaporation of 
wastewater. 

One proprietary treatment system, which among its many 
manifestations has used both FWS-like and VSB-like treat¬ 
ment units as part of its treatment train, is known as the 
Advanced Ecologically Engineered System (AEES), or 
“Living Machine.” This system incorporates conventional 
treatment system components, including sedimentation/ 
anaerobic bioreactors, extended aeration, clarifiers, fixed- 
film reactors, and a final clarifier, sometimes with a VSB 
for polishing, in a greenhouse setting. The AEES was ap¬ 
plied to four demonstration projects funded with federal 
grants. The four projects underwent evaluation of treat¬ 
ment performance for various wastewater types and set¬ 
tings (e.g., raw wastewater in a moderate climate, raw 
wastewater at higher flow rates in a colder climate, in situ 
water-quality improvements to pond water, and polishing 


19 





of secondary effluent). One of the demonstration projects 
also was evaluated by an independent firm under contract 
to the U.S. EPA (EPA, 1997b). Results of performance 
evaluations indicated that wastewater treatment met per¬ 
formance goals for certain wastewater constituents; other 
goals were unmet. Although this technology is presented 
by its developers as a type of natural system, the use of 
wetland plants appears to influence aesthetics more than 
treatment performance. The reader is directed to other 
sources for further information (EPA, 1993b; EPA, 1997b; 
Living Technologies, 1996; Reed et al., 1995; Todd and 
Josephson, 1994). 

Floating macrophyte systems rely only partially on treat¬ 
ment processes provided by wetlands and require mecha¬ 
nized components to achieve the intended treatment per¬ 
formance. Larger duckweed systems and water hyacinth 
systems utilize mechanical systems to remove floating 
macrophytes. Both have been employed to treat waste- 
water by removing some of the wastewater constituents, 
primarily BOD and TSS. In both systems, removal of plants 
usually requires additional mechanical systems for drying, 
disposal, and other residuals handling (Zirschky and Reed, 
1988). 

2.9 Frequently Asked Questions 

1. What are constructed wetlands? 

The term “constructed wetlands” refers to a technol¬ 
ogy designed to employ ecological processes found 
in natural wetland ecosystems. These systems uti¬ 
lize wetland plants, soils, and associated microorgan¬ 
isms to remove contaminants from wastewater. As 
with other natural biological treatment technologies, 
wetland treatment systems are capable of providing 
additional benefits. They are generally reliable sys¬ 
tems with no anthropogenic energy sources or chemi¬ 
cal requirements, a minimum of operational require¬ 
ments, and large land requirements. The treatment 
of wastewater using constructed wetland technology 
also provides an opportunity to create or restore wet¬ 
lands for environmental enhancement, such as wild¬ 
life habitat, greenbelts, passive recreation associated 
with ponds, and other environmental amenities. 

2. What are wetland treatment systems? 

The term “wetland treatment system” generally re¬ 
fers to two types of passive treatment systems. One 
type of system is a free water surface (FWS) con¬ 
structed wetland, which is a shallow wetland with a 
combination of emergent aquatic plants (cattail, bul¬ 
rush, reeds, and others), floating plants (duckweed, 
water hyacinth, and others), and submergent aquatic 
plants (sago pondweed, widgeon grass, and others). 
A FWS wetland may have open-water areas domi¬ 
nated by submergent and floating plants, or it may 
contain islands for habitat purposes. It may be lined 
or unlined, depending on regulatory and/or perfor¬ 
mance requirements. These systems exhibit com¬ 


plex aquatic ecology, including habitat for aquatic 
and wetland birds. 

A second type of system is termed “vegetated sub¬ 
merged bed (VSB)” and is known to many as a sub¬ 
surface flow wetland. A VSB is not an actual wet¬ 
land because it does not have hydric soils. Emer¬ 
gent wetland plants are rooted in gravel, but waste- 
water flows through the gravel and not over the 
surface. This system is also shallow and contains 
sufficiently large gravel to permit long-term subsur¬ 
face flow without clogging. Roots and tubers (rhi¬ 
zomes) of the plants grow into pore spaces in the 
gravel. Most current data indicate that these sys¬ 
tems perform as well without plants as with plants; 
as a result, wetland ecology is not a critical factor 
in VSB systems. 

3. Are constructed wetlands reliable? What do they 
treat? 

Constructed wetlands are an effective and reliable 
water reclamation technology if they are properly 
designed, constructed, operated, and maintained. 
They can remove most pollutants associated with 
municipal and industrial wastewater and stormwater 
and are usually designed to remove contaminants 
such as biochemical oxygen demand (BOD) and 
suspended solids. Constructed wetlands also have 
been used to remove metals, including cadmium, 
chromium, iron, lead, manganese, selenium, zinc, 
and toxic organics from wastewater. 

4. How does a constructed wetland treat wastewa¬ 
ter? 

A natural wetland acts as a watershed filter, a sink 
for sediments and precipitates, and a biogeochemi¬ 
cal engine that recycles and transforms some of 
the nutrients. A constructed wetland performs the 
same functions for wastewater, and a constructed 
wetland can perform many of the functions of con¬ 
ventional wastewater treatment trains (sedimenta¬ 
tion, filtration, digestion, oxidation, reduction, ad¬ 
sorption, and precipitation). These processes oc¬ 
cur sequentially as wastewater moves through the 
wetland, with wastewater constituents becoming 
comingled with detritus of marsh plants. 

5. What is the difference between treatment and en¬ 
hancement wetlands? 

Constructed wetlands generally are designed to 
treat municipal or industrial effluents as well as 
stormwater runoff. Enhancement marshes, or pol¬ 
ishing wetlands, are designed to benefit the com¬ 
munity with multiple uses, such as water reclama¬ 
tion, wildlife habitat, water storage, mitigation banks, 
and opportunities for passive recreation and envi¬ 
ronmental education. Both types of wetland sys¬ 
tems can be designed as separate systems, or 


20 



important attributes of each can be integrated into 
a single design with multiple treatment and en¬ 
hancement objectives. 

6. Can a constructed wetland be used to meet a sec¬ 
ondary effluent standard? 

Both FWS and VSB constructed wetlands can be 
used to meet a 30/30 mg/L BOD and TSS discharge 
standard. It is not advisable to put raw wastewater 
into a constructed wetland. 

7. Can a constructed wetland be used to meet an 
advanced secondary/tertiary discharge standard? 

With sufficient pretreatment and wetland area, FWS 
constructed wetlands can meet discharge standards 
of less than 10 mg/L BOD, TSS, and TN on a 
monthly average basis. Many examples of FWS 
wetland systems meeting these standards on a 
monthly average basis can be found in the United 
States (EPA, 1999). VSB systems have been used 
extensively in England for polishing secondary ef¬ 
fluents and treating effluent from combined sani¬ 
tary and storm sewers. In the U.S., they have gen¬ 
erally not performed well in consistently reaching 
advanced treatment goals with primary treatment 
influent. 

8. How much area is required for constructed wet¬ 
lands? 

As a general rule, a constructed wetland receiving 
wastewater with greater degrees of pretreatment 
(for example, primary clarification, oxidation pond, 
trickling filter, etc.) requires less area than a con¬ 
structed wetland receiving higher-strength waste- 
water. Historically, constructed wetlands designers 
have employed from <2 to over 200 acres/MGD (4 
to 530 L/m 2 -d). However, there is no generic an¬ 
swer to the question since it depends on the efflu¬ 
ent criteria to be met and buffer areas required. 

9. Do these systems have to be lined? 

The requirement for liners in constructed wetlands 
depends on each state’s regulatory requirements 
and/or the characteristics of surface and subsur¬ 
face soils. In a general sense, if soils are porous 
(e.g., sands), well-drained, and contain small 
amounts of loams, clays, and silts, lining is likely to 
be a requirement for constructed wetlands. On the 
other hand, if soils are poorly drained and composed 
mostly of clays, then lining might not be required. 
These systems would tend to produce a layer of 
peat on the bottom that would reduce infiltration with 
time. The concept of a “leaky wetland,” which may 
take advantage of natural processes to purify waste- 
water as it moves downward through soil to re¬ 
charge the ground water, may be considered a po¬ 
tential benefit in certain areas. 


10. What is the role of the plants in constructed wet¬ 
lands? 

In FWS constructed wetlands, plants play several 
essential roles. The most important function of 
emergent and floating aquatic plants is providing a 
canopy over the water column, which limits produc¬ 
tion of phytoplankton and increases the potential 
for accumulation of free-floating aquatic plants (e.g., 
duckweed) that restrict atmospheric reaeration. 
These conditions also enhance reduction of sus¬ 
pended solids within the FWS constructed wetland. 
Emergent plants play a minor role in taking up ni¬ 
trogen and phosphorus. The effect of litter fall from 
previous growing seasons as it moves through the 
water column and eventually decomposes into hu¬ 
mic soil and lignin particles may be significant in 
terms of effluent quality. 

The role of plants in VSB systems is not clear. Ini¬ 
tially it was believed that translocation of oxygen 
by plants was a major source of oxygen to microbes 
growing in the VSB media, and therefore plants 
were critical components in the process. However, 
side-by-side comparisons of planted and unplanted 
systems have not confirmed this belief. Neverthe¬ 
less, planted VSB systems are more desirable aes¬ 
thetically than unplanted horizontal rock-filter sys¬ 
tems, and plants do not appear to hinder perfor¬ 
mance of VSB systems. 

11. How much time is needed for a constructed wet¬ 
land to become fully operational and meet discharge 
requirements? 

For FWS wetland systems, several growing sea¬ 
sons may be needed to obtain the optimum veg¬ 
etative density necessary for treatment processes. 
The length of this period is somewhat dependent 
on the original planting density and the season of 
the initial planting. Effluent quality has been ob¬ 
served to improve with time, suggesting that veg¬ 
etation density and accumulated plant litter play an 
important role in treatment effectiveness. 

VSB systems also require more than one growing 
season to achieve normal wetland plant densities. 
However, the time required for VSB systems to 
become fully operational is considerably less than 
FWS systems because of the minor role of plants 
in the treatment process. Development of the mi¬ 
crobial biomass in the media of a VSB system typi¬ 
cally requires from three to six months. 

12. How long can a FWS wetland operate before ac¬ 
cumulated plant material and settled solids need to 
be removed? 

FWS wetland systems receiving oxidation pond 
effluent may operate for 10 to 15 years without the 
need to remove accumulated litter and settled 


21 





nondegradable influent solids. Treatment capaci¬ 
ties of these wetlands have not shown a decrease 
in treatment effectiveness with time. However, it is 
assumed that further experience will reveal that 
there is a finite period of accumulation that will re¬ 
sult in the need to remove solids. In both types of 
systems, the bulk of the solids accumulation oc¬ 
curs at the influent end of the system. As a result, 
solids may need to be removed from only a portion 
of the system that may be as small as 10 to 25% of 
the surface area. 

13. How much effort is required to operate and main¬ 
tain a constructed wetland? 

These systems require a minimum of operational 
control. Monthly or weekly inspection of weirs and 
weekly sampling typically are required at the efflu¬ 
ent end, and periodic sampling between multiple 
cells is recommended. 

Maintenance of constructed wetlands generally is 
limited to the control of unwanted aquatic plants 
and control of disease vectors, especially mosqui¬ 
toes. Harvesting of plants generally is not required, 
but annual removal or thinning of vegetation or re¬ 
planting of vegetation may be needed to maintain 
flow patterns and treatment functions. 

Effective vector control can be achieved by appro¬ 
priately applying integrated pest management prac¬ 
tices, such as introducing mosquito fish or provid¬ 
ing habitat for mosquito-eating birds and bats. Bi¬ 
monthly monitoring of mosquito larvae and pupae 
and applications of larvacides may be required on 
an as-needed basis. 

Sediment accumulation typically is not a problem 
in a well-designed and properly operated con¬ 
structed wetland, thus partial dredging is required 
only rarely. 

These tasks would require approximately one day 
per week of labor for a wetland system treating a 
flow of one million gallons per day (MGD) (3,880 
m 3 /d) or less, and monitoring may be the most de¬ 
manding task. 

14. Do constructed wetlands produce odors? 

Conventional wastewater treatment processes pro¬ 
duce odors mostly associated with anaerobic de¬ 
composition of human waste and food waste found 
in sewage. These odors usually are concentrated 
in areas of small confinement and point discharges, 
like influent pump stations, anaerobic digesters, and 
sludge-handling processes. Wetlands, in contrast, 
incorporate normal processes of decomposition 
over a relatively large area, potentially diluting odors 
associated with the natural decomposition of plant 
material, algae, and other biological solids. How¬ 


ever, wetland treatment systems receiving septic 
tank and primary effluents can release anaerobic 
odors around the inlet piping, and both types are 
generally anaerobic, which makes odor generation 
a major operational concern. 

15. Are mosquitoes a potential problem with con¬ 
structed wetlands? If so, how are they managed? 

Mosquitoes generally are not a problem in properly 
designed and operated VSB systems. However, 
mosquitoes can be a problem in FWS constructed 
wetlands. If a FWS wetland is designed with suffi¬ 
cient open water (40 to 60% of the surface area) to 
permit effective control with mosquito fish, and in¬ 
let and outlet weirs are placed to allow level control 
and drainage of wetland cells, the potential for 
mosquito populations to thrive is reduced. This lat¬ 
ter concept provides for isolation of various wet¬ 
land cells to allow them to be drained and/or to al¬ 
low predators and mosquitoes to become concen¬ 
trated in pools and channels. 

Along with these physical factors, the development 
of a balanced ecosystem that includes other aquatic 
invertebrates (beetles), aquatic insects (dragon flies 
and damsel flies), fish (top-feeding minnows, stick¬ 
lebacks, gobis, and others), birds (swallows, ducks, 
and others), and mammals (bats) will help main¬ 
tain acceptable levels of mosquitoes. Under these 
conditions, the mosquito is simply a component in 
a balanced food web. If an imbalance develops, 
then intervention with certain biological and chemi¬ 
cal agents may be required. 

A successful intervention method has been the use 
of Bti, a bacterium spore that interferes with devel¬ 
opment of the adult. In essence, Bti kills the larva 
via physical actions. Several applications over the 
mosquito season are needed to interfere with the 
mosquito’s natural growth cycle, which may be three 
to four months in length. Other larvacides, such as 
methoprene, are chemicals that are not selective 
for certain stages of mosquitoes’ life cycle. 
Adulticides also are not selective for life cycles but 
could be used at critical times. 

In general, proper design that supports a healthy 
wetland ecosystem produces conditions that main¬ 
tain sufficiently low mosquito populations. 

16. What is the present level of application of this tech¬ 
nology? 

As of late 1999, more than 200 communities in the 
United States were reported to be utilizing con¬ 
structed wetlands for wastewater treatment. Most 
of these communities use wetlands for polishing 
lagoon effluent. In addition, communities in a wide 
range of sizes use this technology, including large 
cities such as Phoenix, Arizona, and Orange 


22 



County, Florida. For the most part, however, FWS 
technology has been utilized by small- to medium¬ 
sized communities ranging from 5,000 to 50,000 in 
population. 

Even though constructed wetlands for municipal 
wastewater treatment have been around for as long 
as 40 years, there have been widespread problems 
in their performance with respect to nitrogen trans¬ 
formations and removal as well as phosphorus re¬ 
moval (WRC, 2000). This manual has been cre¬ 
ated to help future owners and designers avoid 
unrealistic expectations from these systems. 

17. Can these systems operate at elevations other than 
sea level? 

FWS and VSB systems are found in a wide range 
of elevations extending, for example, from the 
desert Southwest to New England, and from the 
southeast United States to the Rocky Mountains 
and Pacific coast regions. The common wetland 
plants used in these systems are found in all areas 
of the United States and Canada. There is no in¬ 
herent biological or ecological basis for these types 
of systems to not work in the normal range of physi¬ 
ographic conditions in the United States, Canada, 
and Mexico. 

18. Can constructed wetlands work in cold tempera¬ 
tures? 

Constructed wetlands are found in a wide range of 
climatological settings, including cold climates 
where ice forms on the surface for four to six months 
of the year. For example, these systems are found 
in Canada, North Dakota, Montana, Vermont, Colo¬ 
rado, and other cold-climate areas. Special con¬ 
siderations must be included in the design of these 
systems for the formation of an ice layer and the 
effect of cold temperatures on mechanical systems, 
such as the influent and effluent works. The ab¬ 
sence of living plants that have died back for the 
winter and the presence of a layer of ice approxi¬ 
mately 0.5 to 1.0 ft. thick have not been shown to 
severely affect the secondary treatment capabili¬ 
ties of these systems. Nitrogen transformation and 
removal is, however, impaired during very cold pe¬ 
riods. 

19. Can you receive full treatment benefits from a con¬ 
structed wetland that also provides ancillary ben¬ 
efits such as wildlife habitat? 

Multiple benefits can accrue from a FWS con¬ 
structed wetland if it is properly sited and designed. 
For example, FWS wetlands that have a significant 
portion of surface area occupied by submergent 
aquatic plants and deeper water have been found 
to produce higher-quality effluent and provide 
greater habitat value than other configurations. This 


open space is used by aquatic fowl for feeding, 
access to refugia, and as a source of fresh water. 
The same submerged aquatic plants that provide 
wastewater treatment also serve as a food source 
for aquatic birds and mammals. 

Because reduced human health risk is associated 
with tertiary treatment or “polishing” wetlands, they 
have commonly enjoyed full recreational access to 
the FWS systems, but they provide minimal removal 
of several key pollutants in comparison to the treat¬ 
ment wetlands that are the focus of this manual. 
Therefore, human access to these systems entails 
greater health risks because the wastewater is ac¬ 
tively being treated. The wildlife and other natural 
ecological populations may be equally abundant in 
these systems as in the polishing systems, but hu¬ 
man access may be restricted, at least in the inlet 
environs. 

The potential for ancillary benefits is reduced with 
VSB systems. Depending on its size and degree of 
vegetation, a VSB system could provide wildlife 
habitat. VSB wetlands also can be used for envi¬ 
ronmental education and awareness activities. 

2.10 Glossary 

Abiotic Nonbiological processes or treatment mechanisms 
in a constructed wetland. 

Adsorption Adherence by chemical or physical bonding 
of a pollutant to a solid surface. 

Adventitious roots provide a competitive advantage to a 
plant by growing from stems into the surrounding air 
(around terrestrial plants) or water (around aquatic plants) 
before entering the soil substrate to provide additional up¬ 
take or absorption directly from the surrounding medium. 

Aerenchymous tissues in aquatic plants provide for trans¬ 
fer of gases within a plant. In wastewater treatment sys¬ 
tems, emergent aquatic plants rely on aerenchymous tis¬ 
sues for transfer of oxygen to their roots. 

Aerobic processes in wastewater treatment systems take 
place in the presence of dissolved oxygen. 

Algae are single-celled to multicelled organisms that rely 
on photosynthesis for growth. Most algae are classified as 
plants. 

Anaerobic processes in wastewater treatment systems 
take place in the absence of dissolved oxygen and instead 
rely on molecular oxygen available in decomposing com¬ 
pounds. 

Aspect ratio The length of a constructed wetland divided 
by its width (LVW). 

Atmospheric reaeration introduces atmospheric oxygen 
into the water at the water’s surface, which provides dis¬ 
solved oxygen to the aquatic environment. 


23 





Autotrophic Types of reactions that generally require only 
inorganic reactants; for example, nitrification. 

Biochemical oxygen demand (BOD) is the demand for 
dissolved oxygen that decomposition of organic matter 
places on a wastewater treatment process. BOD as ex¬ 
pressed in milligrams per liter (mg/L) is used as a mea¬ 
sure of wastewater organic strength and as a measure of 
treatment performance. This constituent is represented 
throughout the text of this manual as “BOD,” which stands 
for the U.S. standard 5-day BOD test result. 

Biomass is the total amount of living material, including 
plants and animals, in a unit volume. 

Biotic is a term which implies microbiological or biological 
mechanisms of treatment. 

BOD removal is the lowering of demand for dissolved oxy¬ 
gen required for biological decomposition processes in the 
water column; hence, BOD removal can be accomplished 
by biological decomposition in open-water zones and by 
flocculation and sedimentation in fully vegetated zones and 
in VSBs. 

Bulrush is the common name for a number of plants of 
the genus Scirpus found in wetlands. Several species of 
bulrush commonly used in constructed wetlands thrive in 
the wide range of environmental conditions in constructed 
wetlands, including varying levels of water depth and qual¬ 
ity. The large, terete bulrush species include S. validus, S. 
californicus, and S. acutus, all of which form dense stands 
with large numbers of round-sectioned stems that main¬ 
tain an upright posture for one or more years. Other spe¬ 
cies of Scirpus include the three-square varieties, such as 

S. americanus (olynei), S. fluviatilis, and S. robustus, which 
offer tolerance to salinity, a variety of color shades, and 
attractiveness to various animal species. 

Canopy Uppermost or tallest vegetation in a plant com¬ 
munity. 

Cattail is the common name for a number of plants of the 
genus Typha that are common in constructed wetlands in 
the United States, with at least three species predominant: 

T. latifolia, T. domingensis, and T. angustifolia. Along with 
their hybridized forms, these species occupy numerous 
water-depth and water-quality niches within constructed 
wetlands. The wetland designer is advised to consult local 
botanists and geographic references to determine which 
local cattail species or hybrid is best adapted to the spe¬ 
cific water quality, water depth, and substrate planned for 
a constructed wetland. 

Common reed (Phragmites) probably is the most widely 
used plant in constructed wetlands on a worldwide basis, 
but it typically is not used in the United States. Although 
this plant has excellent growth characteristics in very shal¬ 
low constructed wetlands, it is an invasive species in some 
natural wetlands, and its transport and intentional intro¬ 
duction to some localities are discouraged. Common reed 


is considered to offer little value as food or habitat for wet¬ 
land wildlife species (Thunhorst, 1993). 

Constructed wetlands are wastewater treatment systems 
that rely on physical, chemical, and biological processes 
typically found in natural wetlands to treat a relatively con¬ 
stant flow of pretreated wastewater. 

Deciduous Woody plants that shed their leaves in cold 
seasons. 

Dentrification Biotic conversion of nitrate-nitrogen to ni¬ 
trogen gases. 

Detritus Loose, dead leaves and stems from dead veg¬ 
etation. 

Dike A wall of mounded soil that contains or separates 
constructed wetlands from surrounding areas. Dissolved 
oxygen (DO) is required in the water column of a waste- 
water treatment system for aerobic biochemical processes 
that take place in constructed wetlands. 

Dominant plant species The plant species that exerts a 
controlling influence on the function of the entire plant com¬ 
munity. 

Duckweed Duckweed naturally moves on a large water 
surface by movement induced by wind action unless it is 
protected from the wind and held in place by dense stands 
of emergent plants (e.g., macrophytes) or artificial barri¬ 
ers. In FWS systems, this results in dense growths of duck¬ 
weed within the fully vegetated zones. Duckweed effec¬ 
tively seals the water surface and prevents atmospheric 
reaeration. This action combined with the inherent oxygen 
demand of the incompletely treated municipal wastewater 
results in anaerobic conditions in these fully vegetated 
zones. 

Emergent herbaceous wetland plants grow rooted in the 
soil, with plant structures extending above the surface of 
the water. These plants are herbaceous by virtue of their 
relatively decomposable (leafy) plant structures, but they 
also have sufficient internal structure to maintain their up¬ 
right growth, even without the support of surrounding wa¬ 
ters. Most emergent wetland plants grow with or without 
the presence of surface water; however, they generally 
grow in shallow water near the banks of a water body. 

Emergent vegetation (see macrophytes). 

Evapotranspiration Loss of water to the atmosphere 
through water surface and vegetation. 

Exotic species A plant not indigenous to the region. 

Fecal coliform A common measure for pathogenicity of 
wastewater. This analytical test reveals the number of these 
types of organisms in counts/100 milliliters (#/100mL) 

Filtration is the process of filtering influent solids from the 
wastewater and typically is provided by plant stems and 
leaves and other vegetation in the water column. 


24 




Floating aquatic plants are commonly found in FWS sys¬ 
tems, including water hyacinth ( Eichhornia crassipes), 
duckweed ( Lemna spp., Spirodela spp., and Wolffia spp.), 
water fern ( Azolla carotiniana and Salvinia rotundifolia), 
and water lettuce ( Pistia stratiotes). Also common are 
rooted plants growing in a floating form, including penny¬ 
wort ( Hydrocotlyle spp.), water lily ( Nymphaea spp.), frog's 
bit (Limnobium spongia), spatterdock (Nuphar spp.), and 
pondweed ( Potemogeton spp.). 

Floating aquatic systems are in essence shallow basins 
covered with floating aquatic plants. One type of plant that 
can remain in place is the water hyacinth, but it is very 
sensitive to other than tropical temperatures and is con¬ 
sidered to be an invasive species. Duckweed has been 
held in place with artificial barriers in these types of sys¬ 
tems. 

Flocculation is the process of very small particles of mat¬ 
ter clumping together to reach a collectively larger size. In 
wastewater treatment processes, flocculation typically ag¬ 
glomerates colloidal particulates into larger, settleable sol¬ 
ids that are then removed by sedimentation processes. 

Free water surface (FWS) wetlands are constructed wet¬ 
lands that provide wastewater treatment through floccula¬ 
tion and sedimentation during the flow of wastewater 
through stands of aquatic plants growing in shallow water. 
In some FWS wetlands, there are also open areas where 
aerobic bio-oxidation complements the physical removal 
processes. FWS systems resemble natural wetlands in 
function and appearance. FWS systems have also been 
termed “surface flow systems.” 

Function refers to the purpose, role, or actions expected 
of constructed wetlands in the process of wastewater treat¬ 
ment. Function is expressed in terms of expected results, 
such as nutrient uptake, removal of TSS and BOD, main¬ 
tenance of dissolved oxygen in open water zones, and 
reduction of wastewater constituents to acceptable levels, 
waterfowl habitat, and water storage. 

Habitat value The suitability of an area to support a given 
species. 

Herbaceous Plant material that has no woody parts. 

Herbivores are members of the animal kingdom that con¬ 
sume plant matter. 

Hydric soils, or wetland soils, exhibit distinct chemical and 
physical changes that result from periodic inundation and 
saturation. Flooding and subsequent decomposition and 
oxidation of soil chemicals typically result in anaerobic soil 
conditions. 

Hydrophyte Any plant growing in a soil that is deficient in 
oxygen. 

Indigenous species Species of plants that are native to 
an area. 

Inorganics Compounds that do not contain organic car¬ 
bon. 


Lagoons are also called stabilization ponds, oxidation 
ponds, etc. In conventional wastewater treatment systems, 
they typically are used to provide intermediate treatment 
of wastewater through a variety of physical, chemical, and 
biological processes. 

Limiting nutrient is the nutrient that controls a particular 
plant’s growth. When present in insufficient amounts rela¬ 
tive to a given plant’s needs, a limiting nutrient limits that 
plant’s growth. 

Limnetic The open water zone of a FWS system where 
light can penetrate to induce photosynthesis. 

Macrophytes are plants that are readily visible to the un¬ 
aided eye and include vascular or higher plants. Vascular 
plants include mosses, ferns, conifers, monocots, and di¬ 
cots. Macrophytes also may be categorized by a variety of 
ecological growth forms. 

Marsh A common term applied to treeless wetlands. 

Microbes or microorganisms are microscopic organisms 
(only viewed with a microscope), such as bacteria, proto¬ 
zoans, and certain species of algae, which are respon¬ 
sible for many of the biochemical transformations neces¬ 
sary in wastewater treatment processes. 

Nitrification Biotic conversion of ammonium nitrogen to 
nitrite and nitrate-nitrogen. 

Nuisance species Plants that detract from or interfere with 
the designated purpose(s) of constructed wetlands. 

On-site constructed wetland systems are wastewater 
systems for treatment and disposal at the site where waste- 
water is generated. For example, a residential septic sys¬ 
tem is an on-site system. 

Organics Compounds that contain organic carbon (also 
volatile solids). 

Oxygen demand Generally expressed through relatively 
high BOD concentrations, the property of municipal waste- 
water that removes dissolved oxygen from the water col¬ 
umn. 

Photosynthesis is the conversion of sunlight into organic 
matter by plants through a process of combining carbon 
dioxide and water in the presence of chlorophyll and light, 
which releases oxygen as a by-product. 

Phytoplankton are algae that are microscopic in size 
which float or drift in the upper layer of the water column 
and depend on photosynthesis and the presence of phos¬ 
phorus and nitrogen in the water. 

Pneumatophores are structures that provide air channels 
for emergent plants growing in water environments. 

Polishing wetlands are designed to provide tertiary treat¬ 
ment to secondary effluent to meet performance standards 


25 




required by National Pollutant Discharge Elimination Sys¬ 
tem (NPDES) permits. Design considerations for polish¬ 
ing wetlands are outside the scope of this manual. 

Primary effluent is the product of primary treatment of 
wastewater that typically involves settling of solids in a 
containment structure, such as a septic tank, settling pond, 
or lagoon. 

Primary production is the production of biomass (organic 
carbon) by plants and microscopic algae, typically through 
photosynthesis, as the first link in the food chain. 

Primary treatment of wastewater is a settling process for 
removal of settleable solids from wastewaters. 

Rhizome Root-like stem that produces roots and stems to 
propogate itself in a surrounding zone. 

Secondary effluent is wastewater that has undergone sec¬ 
ondary treatment and is discharged to the environment or 
receives further treatment in tertiary treatment processes. 

Secondary treatment continues the process begun in pri¬ 
mary treatment by removing certain constituents, such as 
biochemical oxygen demand (BOD) and total suspended 
solids (TSS) from primary effluent to prescribed treatment 
levels; typically, 30 mg/L in the United States. 

Sediment Organic and mineral particulates that have 
settled from the overlying water column (also sludge). 

Seepage Loss of water from a constructed wetland to the 
soil through infiltration below the system. Senescence is 
the phase at the end of a plant’s life that leads to death 
and, finally, decay. 

Solar insolation refers to the amount of solar radiation 
that reaches the constructed wetland. Solar radiation in 
the summer months may play an important role in photo¬ 
synthesis in open-water zones of a FWS system. 

Standing biomass in a constructed wetland is the total 
amount of plant material that stands erect. This term typi¬ 
cally is used as “dead standing biomass” to refer to dead, 
standing plants, in contrast to green plants and plant litter 
composed of broken and fallen dead plant parts. 

Structure refers to the form and amount of living and 
nonliving components of an ecosystem. For example, 
emergent vegetation provides the structure to perform 
wetland functions. Wetland structure is expressed in 
qualitative terms such as species of flora and fauna, or 
type of wetland such as marsh, bog, or bottomland for¬ 
est. 

Submerged aquatic plants or submergent vegetation 

are rooted plants that grow in open water zones within 
the water column of an aquatic environment (compare to 
emergent aquatic plants) and provide dissolved oxygen 
for aerobic biochemical reactions. They lie below the wa¬ 
ter surface, except for flowering parts in some species. 


Subsurface flow (SF) wetlands (see vegetated sub¬ 
merged bed (VSB) systems). 

Tertiary treatment (see polishing wetlands). 

Total nitrogen (TN) is the sum of all the forms of nitrogen, 
including nitrate, nitrite, ammonia, and organic nitrogen in 
wastewater, and is typically expressed in milligrams per 
liter (mg/L). 

Total phosphorus (TP) is a measure of all forms of phos¬ 
phorus in wastewater, typically expressed in milligrams per 
liter (mg/L). 

Total suspended solids (TSS) are particulate matter in 
wastewater consisting of organic and inorganic matter that 
is suspended in the water column. The numeric value is 
provided by specific analytical test. Typically, municipal 
wastewaters include the settleable solids and some por¬ 
tion of the colloidal fraction. 

Vascular plant Plant that readily conducts water, miner¬ 
als and foods throughout its boundaries. 

Vegetated submerged bed (VSB) systems provide waste- 
water treatment in filter media that is not directly exposed 
to the atmosphere but may be slightly influenced by the 
roots of surface vegetation. VSB systems also have been 
termed subsurface flow (SF) wetlands, rock reed filters, 
submerged filters, root zone method, reed bed treatment 
systems, and microbial rock plant filters. In this manual, 
the term “vegetated submerged bed systems” is used be¬ 
cause gravel beds rather than hydric soils are the support 
media for wetland plants; as a result, the systems are not 
truly wetlands. 

Vegetative reproduction is the process of asexual repro¬ 
duction, in which new plants develop from roots, stems, 
and leaves of the parent plant. 

Wastewater treatment is the process of improving the 
quality of wastewater. The term can refer to any parts 
or all parts of the process by which raw wastewater is 
transformed through biological, biochemical, and physi¬ 
cal means to reduce contaminant concentrations to pre¬ 
scribed levels prior to release to the environment. A 
wastewater treatment process typically consists of pri¬ 
mary, secondary, and tertiary treatment. 

Wetland hydraulics refers to movement of water 
through constructed wetlands, including volumes, 
forces, velocities, rates, flow patterns, and other char¬ 
acteristics. 

Woody plants are plants that produce bark and vascu¬ 
lar structures that are not leafy in nature. Woody plants 
have trunks, stems, branches, and twigs that allow them 
to occupy a greater variety of available niches than her¬ 
baceous plants can occupy. General terms that describe 
categories of woody plants found in wetlands are shrubs, 
trees (canopy or subcanopy), and woody vines. 


26 



Wrack Plant debris carried by water. 

Zooplankton Microscopic and small animals that live in 
the water column. 

2.11 References 

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Armstrong, W. 1978. Root aeration in the wetland envi¬ 
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Bavor, H.J., D.J. Roser, P.J. Fisher, and I.C. Smalls. 
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ment, municipal, industrial, and agricultural. 
Chelsea, Ml: Lewis Publishers, Chapter 39k, pp. 
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Burgan, M.A. and D.M. Sievers. 1994. On-site treatment 
of household sewage via septic tank and two- stage 
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84. 

Burgoon, P.S., K.R. Reddy, and T.A. DeBusk. 1989. Do¬ 
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27 




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28 




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ter treatment in South Africa. 

WaterWorld. 1996. Product focus, June. 


Weller, M.W. 1978. Management of freshwater marshes for 
wildlife. In: R.E. Good, D.F. Whigham, and R.L. Simpson 
(eds.) Freshwater wetlands: Ecological processes and 
management potential. New York, NY: Academic Press, 
pp. 267-284. 

White, K.D. and C.M. Shirk. 1998. Performance and design 
recommendations for on-site wastewater treatment us¬ 
ing constructed wetlands. In: Proceedings of the Eighth 
National Symposium on Individual and Small Commu¬ 
nity Sewage Systems. American Society for Agricultural 
Engineers, Orlando, FL, pp. 195-201. 

Williams, C.R., R.D. Jones, and S.A. Wright. 1996. Mosquito 
control in a constructed wetland. In: Proceedings, 
WEFTEC ‘96, 69th Annual Conference and Exposition, 
Dallas, TX. Water Environment Federation, Alexandria, 
VA, October 1996, pp. 333-344. 

Wittgren, H.B. and T. Maehlum. 1996. Wastewater con¬ 
structed wetlands in cold climates. In: R. Haberl, R. 
Perfler, J. Laber, and P. Cooper (eds.) Water Science & 
Technology 35(5), Wetland systems for water pollution 
control 1996. Oxford, UK: Elsevier Science Ltd., pp. 45- 
53. 

Wolverton, B.C. 1987. Aquatic plants for wastewater treat¬ 
ment: An overview. In: K.R. Reddy and W.H. Smith (eds.) 
Aquatic plants for water treatment and resource recov¬ 
ery. Orlando, FL: Magnolia Publishing, pp. 3-15. 

Worrall, P, K.J. Peberdy, and M.C. Millett. 1996. Constructed 
wetlands and nature conservation. In: R. Haberl, R. 
Perfler, J. Laber, and P. Cooper (eds.) Water Science & 
Technology 35(5), Wetland systems for water pollution 
control 1996. Oxford, UK: Elsevier Science Ltd., pp. 205- 
213. 

Zirschky, J. and S.C. Reed. 1988. The use of duckweed for 
wastewater treatment. Journal of the Water Pollution 
Control Foundation, 60:1253-1258. 


29 





Chapter 3 

Removal Mechanisms and Modeling Performance of 

Constructed Wetlands 


3.1 Introduction 

Constructed wetlands are highly complex systems that 
separate and transform contaminants by physical, chemi¬ 
cal, and biological mechanisms that may occur simulta¬ 
neously or sequentially as the wastewater flows through 
the system. In a qualitative sense, the processes that oc¬ 
cur are known, but in only a few cases have they been 
adequately measured to provide a more quantitative as¬ 
sessment. The predominant mechanisms and their se¬ 
quence of reaction are dependent on the external input 
parameters to the system, the internal interactions, and 
the characteristics of the wetland. The external input pa¬ 
rameters most often of concern include the wastewater 
quality and quantity and the system hydrological cycle. 

Typical characteristics of municipal wastewaters most 
often treated in constructed wetlands are described in Table 
3-1. The emphasis of this manual is on the treatment of 
municipal wastewater with the objectives of achieving tar¬ 
get levels of suspended solids, organic matter, pathogens, 
and in some instances, nutrients (specifically total nitro¬ 
gen) and heavy metals. Wastewaters that will be consid¬ 
ered include septic tank effluent, primary effluent, pond 
effluents, and some secondary effluents from overloaded 
or poorly controlled systems. Table 3-1 shows that the char¬ 
acter of the wastewater is dependent on pretreatment and 


Table 3-1. Typical Constructed Wetland Influents 


Constituent 

(mg/L) 

Septic Tank 
Effluent 1 

Primary 

Effluent 2 

Pond 

Effluent 3 

BOD 

129-147 

40-200 

11-35 

Sol. BOD 

100-118 

35-160 

7-17 

COD 

310-344 

90-400 

60-100 

TSS 

44-54 

55-230 

20-80 

VSS 

32-39 

45-180 

25-65 

TN 

41-49 

20-85 

8-22 

nh 3 

28-34 

15-40 

0.6-16 

no 3 

0-0.9 

0 

0.1-0.8 

TP 

12-14 

4-15 

3-4 

OrthoP 

10-12 

3-10 

2-3 

Fecal coli (log/100ml) 

5.4-6.0 

5.0-7.0 

0.8-5.6 


'EPA (1978), 95% confidence interval. Prior to major detergent 
reformulations which reduce P species by -50%. 

2 Adapted from Metcalf and Eddy, (1991) assuming typical removal by 
primary sedimentation-soluble BOD = 35 to 45% total. 

3 EPA (1980). 


may contain both soluble and particulate fractions of or¬ 
ganic and inorganic constituents in reduced or oxidized 
forms. As will be seen later, these characteristics play an 
important role in the major mechanisms of removal. 

The two major mechanisms at work in most treatment 
systems are liquid/solid separations and constituent trans¬ 
formations. Separations typically include gravity separa¬ 
tion, filtration, absorption, adsorption, ion exchange, strip¬ 
ping, and leaching. Transformations may be chemical, in¬ 
cluding oxidation/reduction reactions, flocculation, acid/ 
base reactions, precipitation, or a host of biochemical re¬ 
actions occurring under aerobic, anoxic, or anaerobic con¬ 
ditions. Both separations and transformations may lead to 
contaminant removal in wetlands but often only result in 
the detention of the contaminant in the wetland for a pe¬ 
riod of time. There may be changes in the contaminant 
composition that will effectively achieve treatment objec¬ 
tives, such as the biochemical transformation of organic 
compounds to gases such as C0 2 or CH 4 . A biochemical 
transformation, however, may produce biomass or organic 
acids that may not achieve the treatment objective if these 
materials escape in the effluent. In the case of biomass, it 
may escape as volatile suspended solids, or it may un¬ 
dergo further bacterial reaction, which may result in the 
leaching of a soluble carbon compound back into the wa¬ 
ter column. 

The remainder of this chapter will review potential mecha¬ 
nisms that may be at work in constructed wetlands. These 
reactions may occur in the water column, on the surfaces 
of plants, within the litter and detritus accumulating at the 
wetland surface or on the bottom, or within the root zone 
of the system. The reactions unique to the wetland type 
will also be delineated. 

3.2 Mechanisms of Suspended Solids 
Separations and Transformations 

3.2.1 Description and Measurement 

Suspended solids in waters are defined by the method 
of analysis. Standard Methods (1998) defines total sus¬ 
pended solids as those solids retained on a standard glass 
fiber filter that typically has a nominal pore size of 1.2pm. 
The type of filter holder, the pore size, porosity, area and 
thickness of the filter, and the amount of material depos- 


30 







ited on the filter are the principal factors affecting the sepa¬ 
ration of suspended from dissolved solids. As a result, the 
measurement reported for total suspended solids may in¬ 
clude particle sizes ranging from greater than 100|im to about 
1 pm. Soluble (dissolved) solids would therefore include col¬ 
loidal solids smaller than 1 pm and molecules in true solu¬ 
tion. A classical method of solids classification by size would 
include the following: 

Settleable Solids >100pm 

Supracolloidal Solids 1 -100pm 

Colloidal Solids 10-3-1 pm 

Soluble Solids <10-3pm 

Solids are also classified as volatile or fixed, again based 
on the method of analysis. Standard Methods (1998) de¬ 
fines a volatile solid as one that ignites at 550°C. Although 
the method is intended to distinguish between organic sol¬ 
ids and inorganic solids, it is not precise since volatile solids 
will include losses due to the decomposition or volatilization 


of some mineral salts depending on the time of exposure to 
the ignition temperature. 

Wastewater influents to wetlands may contain significant 
quantities of suspended solids (Table 3-1). The composition 
of these solids is quite different, however. Septic tank and 
primary effluents will normally contain neutral density colloi¬ 
dal and supracolloidal solids emanating from food wastes, 
fecal materials, and paper products. Pond effluent suspended 
solids are likely to be predominantly algal cells. All three will 
be high in organic content. Size distribution is also different 
among the waste streams. Tables 3-2 and 3-3 present infor¬ 
mation on size distributions of suspended solids, organic 
matter, and phosphorus in domestic wastewaters with vari¬ 
ous levels of pretreatment. It should be noted that methods 
differed between investigators on estimating size ranges. 
High settleable fractions are not surprising for raw wastewa¬ 
ter samples or for the pond effluent containing algal cells. It 
is important to note the association of organic matter and 
phosphorus with the various solid fractions. 


Table 3-2. Size Distributions for Solids in Municipal Wastewater 


Type of Sample 
(% by Weight) 


Size Range 
(fxm) 

Primary Eff. 1 

Primary Eft. 2 

Primary Eff. 3 

Raw Sewage 4 

Raw Sewage 5 

<10 3 

_ 

_ 

_ 

31 

48 

10 3 -1.0 

20 

22(10-30) 

- 

14 

9 

1.0-12 

54 

35(24-51) 

- 

- 

- 

>12 

26 

43(30-60) 

- 

- 

- 

1.0-100 

- 

- 

81 

24 

18 

>100 

- 

- 

19 

31 

23 


' Levine et al. (1984). 
2 Tchobanoglous et al. (1983). 
3 Gearheart, etal. (1993). 
4 Heukelekian and Balmat (1959). 
5 Rickert and Hunter ((1972). 


Table 3-3. Size Distribution for Organic and Phosphorus Solids in Municipal Wastewater 

Type of Solids 
(% by Weight) 


Size Range 
(nm) 

Organic 

Solids' 

(primary 

effluent) 

Organic 

Solids 2 

(primary 

effluent) 

Organic 

Solids 2 

(primary 

effluent) 

Organic 

Solids 2 

(primary 

effluent) 

Organic 

Solids 2 

(primary 

effluent) 

Organic 

Solids 3 

(raw 

sewage) 

Total 

Phos. 4 

(primary 

effluent) 

Total 

Phos. 4 

(primary 

effluent) 

<10 3 

9 

_ 

_ 

_ 

_ 

42 

_ 

_ 

<0.1 

- 

51 

50 

25 

35 

- 

17.2 

15.7 

10-M.0 

? 

- 

- 

- 

- 

11 

- 

- 

0.1-1.0 

- 

8 

19 

2 

1 

- 

54.6 

67.0 

1.0-12 

- 

34 

26 

13 

13 

- 

7.1 5 

6 T 

>12 

- 

7 

5 

60 

41 

- 

21.0® 

8.5" 

1.0-100 

15 

- 

- 

- 

- 

20 

- 

- 

>100 

28 

- 

- 

- 

- 

27 

- 

- 


' Munch et al. (1980). 
2 Levine et al. (1991). 

3 Rickert and Hunter (1972). 
“Levine et al. (1984). 

5 Range: 1-5 /urn 
6 Range: >5 //m 


31 













3.2.2 Suspended Solids in Free Water 
Surface Wetlands 

Total suspended solids are both removed and produced 
by natural wetland processes. The predominant physical 
mechanisms for suspended solids removal are flocculation/ 
sedimentation and filtration/interception, whereas suspended 
solids production within the wetland may occur due to death 
of invertebrates, fragmentation of detritus from plants, pro¬ 
duction of plankton and microbes within the water column or 
attached to plant surfaces, and formation of chemical pre¬ 
cipitates such as iron sulfide. Figure 3-1 illustrates the most 
important of these processes as they occur in a FWS sys¬ 
tem. Resuspension of solids may occur due primarily to tur¬ 
bulence created by animals, high inflows, or winds. A brief 
discussion of some of these processes and how they may 
affect free water surface systems follows. 

3.2.2.1 Discrete and Flocculant Settling 

Typically, particulate settling produced by gravity may 
be categorized as discrete or flocculant settling. Both sepa¬ 
ration processes exploit the properties of particle size, spe¬ 
cific gravity, shape, and fluid specific gravity and viscosity. 
Discrete settling implies that the particle settles indepen¬ 


dently and is not influenced by other particles or changes 
in particle size or density. A mathematical expression for 
the terminal settling velocity of the discrete particle may 
be derived from Newton’s Law. Under laminar flow condi¬ 
tions, which exist in fully vegetated zones of a FWS and in 
VSBs, the velocity of a spherical particle can be estimated 
by Stokes’ Law, which states that the settling velocity is 
directly proportional to the square of the nominal diameter 
and the difference in particle and fluid densities and is in¬ 
versely proportional to fluid viscosity. Drag on the particle 
that influences settling velocity is affected by particle shape, 
fluid/particle turbulence, and fluid viscosity. 

Whereas discrete settling can be estimated given the 
independent variables discussed previously, flocculant 
settling cannot be so easily determined, requiring experi¬ 
mental measurement. It occurs as the result of particle 
growth and, perhaps, change in characteristics overtime. 
As a result, particle settling velocity typically increases with 
time. Flocculent settling is promoted by the relative move¬ 
ment of target particles in such a fashion as to cause an 
effective collision. This relative velocity (velocity gradient) 
is often calculated as G, the mean velocity gradient, which 
is a function of power input, dynamic viscosity, and system 
volume (Camp and Stein, 1943). Effective velocity gradi- 


Duckweed, Floating Litter & Detritus 



Settled TSS & Detritus from Plants 



Dissolved Oxygen «0 (Anoxic) 


Fully Vegetated Zone 

Removals due to Flocculation, 
Sedimentation, Adsorption and 
Anaerobic Reactions, Primarily 


Atmospheric 

Reaeration 


-WAr 

BOD Oxidation 
NH 4 -N-^N0 3 -N 



Solar 
Radiation 


Submerged Vegetation 




Dissolved Oxygen +++(Aerobic) 

Open Water Zone 

Transformations by Aerobic Biological 
Treatment, Primarily 

Pathogen Kill by Sunlight & Time 


Figure 3-1. Mechanisms that dominate FWS systems 


32 
































ents for flocculation range from 10 to 75 sec- 1 . Floccula¬ 
tion may occur naturally, as when fresh water flows into 
saline water forming a delta, or it may require chemical 
(coagulant) addition. It may affect large particles (100pm 
to 1000pm) of low to moderate specific gravity (1.001 to 
1.01) and small particles (1.0pm to 10pm) with high spe¬ 
cific gravity (1.5 to 2.5). The formation of larger flocculant 
particles is dependent on the electric charge on the par¬ 
ticle surface. Like electrical charges on the double layer 
surrounding particles may produce particle stability that 
hinders attachment even if collisions take place. This 
charge is sensitive to the composition of the fluid. Adsorp¬ 
tion of solutes to the surface occurs as a result of a variety 
of binding mechanisms, which may eventually result in the 
destabilization of the particles and result in particle adhe¬ 
sion. There has been little work done on the evaluation of 
natural flocculation phenomena with primary effluent or 
algal cells. The existence of emergent plant stems in FWS 
wetlands will promote effective velocity gradients for par¬ 
ticle collisions, but the adhesion of these particles would 
be dependent on surface properties that would be influ¬ 
enced by water column quality. 

In wetland systems treating primary or septic tank efflu¬ 
ents (or secondary effluents), particle sizes are mostly in 
the colloidal to low supracolloidal range (Table 3-2). Typi¬ 
cally sedimentation processes will remove material larger 
than about 50pm with specific gravity of about 1.20. The 
remaining particles are normally the lower density materi¬ 
als. Using Stokes’ Law to approximate discrete settling 
velocity, particles ranging from 1.0pm to 10pm with a spe¬ 
cific gravity ranging from 1.01 to 1.10 will settle at a rate of 
from 0.3 to 4 x 10~ 4 m/d. Typical hydraulic loads to FWS 
wetlands are in the range of 0.01 to 0.5 m/d (note that the 
hydraulic load is equivalent to the mean settling velocity of 
a particle that will be removed exactly at that loading). As¬ 
suming the higher settling velocity of 0.3 m/d and a typical 
FWS system velocity of 50 m/d and depth of 0.8m, the 
larger particles would settle by gravity in approximately 
2.7 days, or 133 m along the wetland longitudinal axis. 
The smaller, less dense particles would require over 200 
days and a length of over 11,000 m. Therefore it can be 
concluded that the larger, denser particles could be re¬ 
moved in the primary zone of a wetland based on simple 
discrete settling theory (see Chapter 4 for more details on 
design). The smaller, neutral-density particles, which make 
up a significant fraction of septic tank and primary effluent, 
are not likely removed in this primary zone by simple dis¬ 
crete sedimentation, but may be flocculated due to the 
velocity gradients imposed by emergent plant stems in the 
water column. It is also possible that some particles may 
be intercepted by angular emergent plant tissue as would 
occur in settling basins equipped with plate or tube set¬ 
tlers. Clearly, removal of TSS by a FWS wetland is more 
complex than predicted by discrete settling theory. There 
is currently insufficient transect data available on waste- 
water influents of interest to develop a rational separation 
model, either qualitatively or quantitatively, for TSS removal 
from primary or septic tank effluents in FWS systems. 


For wetland systems receiving pond effluents, the pri¬ 
mary source of suspended solids for much of the season 
is algal cells. These cells include green algae, pigmented 
flagellates, blue-greens, and diatoms. Sizes range from 
Ip to lOOiim, and shapes may range from coccoid to fila¬ 
mentous. Specific gravity of actively growing algal cells 
may be close to that of water insofar as they must remain 
suspended high up within the water column in order to 
survive. Flotation may be accomplished by gas vacuoles 
(blue-green algae), gelatinous sheathes, or shapes that 
increase particle drag. It is believed that wind-induced tur¬ 
bulence and vertical water motion greatly influence algae 
distribution in ponds (Bella, 1970). Motile algae are not 
typically predominant in wastewater pond systems. Once 
algal cells die for lack of nutrients and/or sunlight, they 
lose this flotation characteristic and will settle. Settling ve¬ 
locities range from 0.0 to 1.0 m/s (typically, 0.1 to 0.3 m/s) 
depending on species and physiological condition 
(Hutchinson, 1967). It is likely that many of these cells will 
be removed by sedimentation in wetlands covered by 
emergent vegetation providing shading and reducing wind 
action. Flocculation of the cells within the wetland is also 
possible although little experimental evidence has been 
presented to date. Table 3-4 was generated by Gearheart 
and Finney (1996), and it represents the only known appli¬ 
cation of the particle-size theory to show that colloidal frac¬ 
tions are flocculated in FWS systems (see Chapter 4 for 
further explanation). Figure 3-2 illustrates the removal of 
TSS observed for a fully vegetated FWS wetland treating 
pond effluent. Attempts to settle algae from ponds in open 
settling basins have not been successful, however, likely 
due to the presence of light and wind action. 

3.2.2.2 Filtration/Interception 

Filtration, in the usual sense of this unit process, is not 
likely to be significant in surface wetlands. Stems from 
emergent plants are too far apart to effect significant en¬ 
trapment of the particle sizes found in influent to these 
wetlands. Furthermore, plant litter and detritus at the sur¬ 
face and bottom of the wetland are high in void fraction 
such that filtration is not likely an important mechanism. 
On the other hand, interception and adhesion of particles 
on plant surfaces could be significant mechanisms for re¬ 
moval. The efficiency of particle collection would be de¬ 
pendent on particle size, velocity, and characteristics of 
the particle and the plant surfaces that are impacted. In 
wetlands, plant surfaces in the water column are coated 
with an active biofilm of periphyton. This biofilm can ad¬ 
sorb colloidal and supracolloidal particles as well as ab¬ 
sorb soluble molecules. Depending on the nature of the 
suspended solids, they may be metabolized and converted 
to soluble compounds, gases, and biomass or may physi¬ 
cally adhere to the biofilm surfaces to eventually be 
sloughed off into the surrounding water column. Similar 
reactions may occur in the surface detritus or at the surficial 
bottom sediment. To date, there have been no definitive 
studies reported on the importance of this mechanism in 
suspended solids removal in free surface wetlands. 

3.2.2.3 Resuspension 

In FWS wetlands, velocity induced resuspension is mini¬ 
mal. Water velocities are too low to resuspend settled par- 


33 




Table 3-4. Fractional Distribution of Bod, COD, Turbidity, and SS in the Oxidation Pond Effluent and Effluent from Marsh Cell 5 (Gearheart and 
Finney, 1996) 




BOD 


COD 


Turbidity 


SS 


mg/I 

% 

mg/I 

% 

NTU 

% 

MG/L 

% 

Oxidation Pond 









Fraction 









Total 

27.5 

100 

80 

100 

11.0 

100 

31.0 

100 

Settleable 

3.7 

13 

5 

6 

2.5 

23 

5.8 

19 

Supracolloidal 

13.7 

50 

23 

29 

5.3 

48 

25.2 

81 

Colloidal, soluble 

10.1 

37 

52 

65 

3.2 

29 

“ 

“ 

Marsh Fraction 









Total 

4.8 

100 

50 

100 

3.9 

100 

2.3 

100 

Settleable 

0 

0 

0 

0 

0.6 

15 

0.3 

13 

Supracolloidal 

1.2 

24 

4 

8 

1.6 

42 

2.0 

87 

Colloidal, soluble 

3.6 

76 

46 

92 

1.7 

43 

- 

“ 



Figure 3-2. Weekly transect TSS concentration for Areata cell 8 pilot receiving oxidation pond effluent (EPA, 1999) 


tides from bottom sediments or from plant surfaces. Fur¬ 
thermore, fully vegetated wetlands provide excellent sta¬ 
bilization of sediments by virtue of sediment detritus and 
root mats. The reintroduction of settled solids in wetlands 
is most likely due to gas-lift in vegetated areas or 
bioturbation or wind-induced turbulence in open water ar¬ 
eas. Wetland sediments and microdetritus are typically near 
neutral buoyancy, flocculant, and easily disturbed. 
Bioturbation by fish, mammals, and birds can resuspend 
these materials and lead to increases in wetland suspended 
solids. The oxygen generated by algae and submerged 
plants, nitrogen oxides and nitrogen gas from denitrifica¬ 
tion, or methane formed in anaerobic process may cause 
flotation of particulates (Kadlec and Knight, 1996). 


As discussed previously, the generation of new biom¬ 
ass by primary production or through the metabolism of 
influent wastewater constituents will eventually result in 
the return of some suspended materials back into the wa¬ 
ter column. The magnitude of wetland particulate cycling 
is large, with high internal levels of gross sedimentation 
and resuspension, and almost always overshadows influ¬ 
ent TSS loading in natural or tertiary treatment wetlands. 
The effluent TSS from a wetland rarely results directly from 
nonremovable TSS in the influent wastewater and is often 
dictated by the wetland processes that generate TSS in 
the wetland. Typical background TSS concentrations ex¬ 
pected in FWS wetlands appear in Table 3-5. It should be 
noted that large expanses of open wetland prior to dis- 


34 
































charge structures could result in unusually high effluent 
TSS concentrations due to the production of excessive 
amounts of algae and induced high levels of wildlife activi¬ 
ties that could produce effluent variations as typified in Fig¬ 
ure 3-3. 

High incoming TSS or organic loading will result in a 
measurable increase in bottom sediments near the inlet 
structure (Van Oostrom and Cooper, 1990; EPA, 1999). 
However, no FWS treatment wetland has yet required 
maintenance because of sediment accumulation, includ¬ 
ing some that have been in service for over 20 years. 

3.2.3 Suspended Solids in Vegetated 
Submerged Beds 

One of the primary intermediate mechanisms in the re¬ 
moval of suspended solids by VSB systems is the floccu¬ 
lation and settling of colloidal and supracolloidal particu¬ 
lates. These systems are relatively effective in TSS removal 
because of the relatively low velocity and high surface area 
in the VSB media. VSBs act like horizontal gravel filters 
and thereby provide opportunities for TSS separations by 
gravity sedimentation (discrete and flocculant), straining 
and physical capture, and adsorption on biomass film at¬ 
tached to gravel and root systems. Clogging of the filter 
media has been of some concern especially with high TSS 
loading, but documentation of this phenomenon has not 
been forthcoming. The accumulation of recalcitrant or 
slowly degradable solids may eventually lead to increased 
headlosses near the influent end of the system. Design 
features to overcome this are described in Chapter 5. 

The importance of vegetation in VSB systems has been 
debated for some time. Several recent studies have com¬ 


pared pollutant removal performance of planted and 
unplanted VSB systems and have shown no significant dif¬ 
ference in performance (Liehr, 2000; Young et al., 2000). 
The importance of plant type has also been evaluated 
(Gersberg et al., 1986; Young et al., 2000). Maximum root 
length and growth rates have been reported. Although some 
investigators claimed that certain treatment goals are likely 
to benefit from certain plants, these claims have not been 
sustained by others (Young et al., 2000). The extension of 
root system within the gravel bed is dependent on system 
loading, plant type, climate, and wastewater characteristics, 
among other variables. It appears that a dominant fraction 
of the flow passes below the root system in VSB facilities. 
The role of root surfaces in TSS removal has not been proven 
experimentally. 

The contributions of internal biological processes to ef¬ 
fluent TSS is likely similar to that found in FWS systems, 
although algal contributions should be negligible. 
Resuspension of separated solids is not likely since sys¬ 
tem velocities are low and scouring should not be signifi¬ 
cant. Furthermore, bioturbation in these systems should 
be minimal. Background concentrations for VSB systems 
have not yet been definitively documented with reliable 
information. 

3.3 Mechanisms for Organic Matter 
Separations and Transformations 

3.3.1 Description and Measurement 

Organic matter in wastewater has been measured in a 
number of ways over the years. Because the organic frac¬ 
tion in wastewater is often complex and the concentra¬ 
tions of the individual components relatively low, analyses 


Table 3-5. Background Concentrations of Contaminants of Concern in FWS Wetland Treatment System Effluents 


Constituent 

Range (mg/L) 

Typical (mg/L) 

Factors Governing Value 

Reference 

TSS 

2-5 

3 

Plant types, coverage, 
Climate, wildlife 

Reed et al.,1995; 

Kadlec and Knight, 1996 

BOD 5 ’ 

2-8 

5 

Plant types, coverage, 
Climate, plant density 

Reed et al., 1995 
Gearheart, 1992 

BOD 5 2 

5-12 

10 

Plant types, coverage, 
Climate, plant density 

Kadlec and Knight, 1996 

TN 

1-3 

2 

Plant types, coverage, 
Climate, oxic/anoxic 

Kadlec and Knight, 1996; 
Reed et al., 1995 

NH -N 

4 

0.2-1.5 

1.0 

Plant types, coverage, 
Climate, oxic/anoxic 

Kadlec and Knight, 1996; 
Reed et al., 1995 

TP 

0.1-0.5 

0.3 

Plant types, coverage, 
Climate, soil type 

Kadlec and Knight, 1996; 
Reed et al., 1995 

Fecal Coli CFU/100 ml 

50-5,000 

200 

Plant types, coverage, 
Climate, wildlife 

Watson et al, 1987; 
Gearheart et al., 1989 


'Wetland system with significant open water and submergent vegetation 
2 Wetland system fully covered by emergent vegetation 


35 











O) 

E 

o 

o 

CD 

** 

c 

<D 

3 

<*- 

«*- 

LU 



July-91 July-92 


July-93 July-94 July-95 July-96 July-97 

Date 


Figure 3-3. Variation in effluent BOD at the Areata enhancement marsh (EPA, 1999) 


are often performed on the aggregate amount of organic 
matter comprising organic constituents with common char¬ 
acteristics. Methods for total organic carbon (TOC) and 
volatile solids (VS) measure the total amount of organic 
matter present. Chemically oxidizable organic matter is 
often measured as Chemical Oxygen Demand (COD), 
expressed in units of oxygen, and biodegradable organic 
matter is determined by the Biochemical Oxygen Demand 
(BOD) procedure. All of these aggregate methods have a 
place in assessing pollutant levels in water, but none will 
provide information on specific organic molecules or their 
fate in treatment processes. As a result, any qualitative or 
quantitative model attempting to express mechanistic be¬ 
havior of organic matter in a system is an empirical model 
based on observation of the parameter of interest. In wet¬ 
lands, physical, chemical, and biochemical reactions will 
transform and/or separate organic matter, often leading to 
different species of organic molecules. Thus the BOD (or 
COD, or TOC) of the influent to a wetland does not mea¬ 
sure the same organic constituents that appear in the ef¬ 
fluent. This phenomenon is no different than what has al¬ 
ready been discussed for TSS. 

Most regulatory agencies today establish wastewater 
discharge permit limits based on BOD values; thus much 
of the data available is expressed as that parameter. It is 
difficult to use BOD in mass balance calculations insofar 
as it is a dynamic measure conducted over a finite time 
(usually five days) and at a specified temperature (usually 
20°C). One can use it to estimate roughly the oxygen re¬ 
quirements for aerobic systems if the rate of oxygen de¬ 
mand exertion (expressed by the biodegradation constant, 
k) is known. To further complicate the use of this param¬ 
eter, the analysis of BOD may or may not include the ni¬ 
trogenous oxygen demand (NOD), which may be ex¬ 
pressed concurrently or simultaneously with the carbon¬ 
aceous oxygen demand. Thus primary influent BOD may 
measure carbonaceous organic matter, whereas effluent 
from the wetland may include both carbonaceous and ni¬ 


trogenous (ammonia) matter depending on the system. 
Table 3-1 presents typical values of total and soluble BOD 
for primary and septic tank effluents as well as lagoon ef¬ 
fluents. Values of COD and VSS (volatile suspended sol¬ 
ids) are also provided. There is no simple way to relate 
BOD and COD values for wastewater insofar as the differ¬ 
ences that exist between degradable and chemically oxi¬ 
dizable fractions. 

3.3.2 Organic Matter in Free Water 
Surface Wetlands 

3.3.2.1 Physical Separations of Organic Matter 

Table 3-4 illustrates that influents typically received by 
FWS systems contain some particulate organic matter. A 
significant amount of the influent from septic and primary 
effluents is in the dissolved and colloidal fraction as would 
be expected, whereas pond effluents may contain a size¬ 
able fraction of supracolloidal material represented by al¬ 
ga! cells. Separation of particulate organic matter would 
occur by the same mechanisms as those described for 
TSS. It is not uncommon to find organic matter removal in 
the influent end of a FWS system that parallels TSS re¬ 
moval. Figure 3-4 illustrates predominant organic matter 
separations and transformations that occur in FWS sys¬ 
tems. As noted in section 3.3.2.2, biochemical transforma¬ 
tions of the entrapped and settled organic matter will greatly 
influence the apparent removal of total organic matter within 
the water column. 

Soluble organic matter may also be removed by a num¬ 
ber of separation processes. Adsorption/absorption (the 
movement of contaminants from one phase to another) is 
an important process affecting some organic molecules. 
The process is often referred to as sorption to cover both 
adsorption and absorption processes in natural systems 
because the exact manner in which partitioning to the sol¬ 
ids occurs is often not known. Partitioning of organic mat¬ 
ter between solids can be understood and predicted to 


36 



















some degree using physiochemical properties of the or¬ 
ganic (e.g., water solubility, octanol-water partition coeffi¬ 
cient). Sorption is often described by an isotherm 
(Freundlich) relationship. The degree of sorption and its 
rate are dependent on the characteristics of both the or¬ 
ganic and the solid. In wetlands, the important solid sur¬ 
faces would include the plant litter and detritus occurring 
at the wetland surface or on the bottom and plant stems 
and leaves often covered by a periphyton biofilm. The sorp¬ 
tion process may be reversible or irreversible depending 
on the organic and solid. Sorbents may eventually become 
“saturated” with sorbate although often the sorbed organic 
is biochemically transformed, renewing the sorbent capac¬ 
ity. Many of the wetland solid surfaces are also renewed 
by continuous turnover of biomass that makes up the ma¬ 
jor component of the sorbent. It is believed that sorption 
processes play an important role in organic separation in 
wetlands, but the processes have not been adequately 
quantified at this time. 

Volatilization may also account for loss of certain organ¬ 
ics. In this reaction, the organic partitions to the air. The 
propensity for a given compound to move from liquid to 
gaseous phase is measured by Henry’s constant. The 
higher the value for a compound, the more likely it will par¬ 
tition to the gaseous phase. Generally, organic matter en¬ 
tering a wetland receiving pretreatment will not contain sig¬ 


nificant quantities of volatile compounds (VOCs). Faculta¬ 
tive lagoons have been shown to remove 80 to 96% of 
volatile organics from municipal wastewater (Hannah et 
al., 1986). However, some of these organic materials may 
be produced by biological transformations. Precipitation 
reactions with organic matter found in wetland influents 
have not been documented for municipal wastewater. 

3.3.2.2 Biological Conversions of Organic 
Matter 

The Biochemistry 

Biochemical conversions are important mechanisms 
accounting for changes in concentration and composition 
of biodegradable organic matter in wetlands. They may 
account for removal of some organic constituents by vir¬ 
tue of mineralization or gasification and the production of 
organic matter through synthesis of new biomass. Organ¬ 
isms will consume organic matter (and inorganic matter 
as well) in order to sustain life and to reproduce. The or¬ 
ganic matter in wastewater serves as an energy source as 
well as a source of molecular building blocks for biomass 
synthesis. The reactions occurring therefore are ones that 
prepare the molecule for use by the organism or are di¬ 
rectly involved in the extraction of energy or incorporation 
of building blocks in the synthesis reaction. End products 
of the reaction are waste products of the system. These 



DIC - Dissolved Inorganic Carbon 
VOC - Volatile Organic Carbon 
PIC - Particulate Inorganic Carbon 


Figure 3-4. Carbon transformations in a FWS wetland 


37 




















































reactions include oxidation/reduction processes, hydroly¬ 
sis, and photolysis. 

Since energy is a key element in the biochemical sys¬ 
tem, the reaction is classified as chemotrophic or pho- 
totrophic depending on whether the reaction utilizes a 
chemical source of energy or light energy. The environ¬ 
ment greatly influences the energetics of the reaction that 
is driven by electron transfers. If the element at the end of 
this set of transfers is oxygen, the reaction is referred to as 
an aerobic reaction. Aerobic metabolism requires dissolved 
oxygen and results in the most efficient conversion of bio¬ 
degradable materials to mineralized end products, gases, 
and biomass. Anoxic (anaerobic respiration) reactions use 
nitrates, carbonates, or sulfates as terminal electron ac¬ 
ceptors (in place of oxygen). Terminal electron acceptors 
are subsequently reduced in these reactions, producing 
end products such as nitrogen oxides, free nitrogen, sul¬ 
fur, thiosulfate, and so forth. These reactions are typically 
less efficient (biomass produced per unit substrate utilized) 
than aerobic reactions and produce mineralized end prod¬ 
ucts and gases, but less biomass per unit of substrate con¬ 
verted. Anaerobic metabolism takes place in the absence 
of dissolved oxygen and uses organic matter as the termi¬ 
nal electron acceptor as well as the electron donor. These 
transformations are the least efficient of the biochemical 
reactions and will not result in the reduction in BOD (or 
COD) unless hydrogen or methane is produced, since elec¬ 
trons released in the oxidation of the organic are passed 
to electron acceptors (reduced products such as alcohols 
and organic acids) that remain in the medium. The only 
energy loss in the system is that due to microbial ineffi¬ 
ciencies. 

The transformations described previously rely on chemi¬ 
cal energy to drive them. A very important transformation 
that takes place in open water areas of FWS wetlands uses 
sunlight energy to drive the reaction (photosynthesis). In 
these reactions, inorganic carbon (C0 2 and carbonates) is 
synthesized to form biomass (primary production), releas¬ 
ing oxygen. Thus both oxygen and organic matter are pro¬ 
duced within the system. 

The previous reactions and their stoichiometry, equilib¬ 
rium, and rates are dependent on environmental variables 
including dissolved oxygen, temperature, oxidation/reduc¬ 
tion potential, and chemical characteristics, to name a few. 
An excellent reference on microbial metabolism in waste- 
water systems can be found in Grady and Lim (1980). 

The Organisms 

Biochemical reactions of importance in FWS constructed 
wetlands are carried out by a large number of organism 
classes. They may be classified based on their position in 
the energy or food chain (producers, consumers, or de¬ 
composers) or on their life form or habitat. A classical de¬ 
scription relative to a wetland habitat would include 

• Benthos—organisms attached or resting on the bot¬ 
tom within the plant litter and detritus or living within 
the sediments 


• Periphyton—both plants and animals attached to stems 
and leaves of rooted plants 

• Plankton—floating plants or animals whose move¬ 
ments are generally dictated by currents 

• Neuston—plants or animals resting or swimming on 
the wetland surface 

• Nekton—swimming organisms able to navigate at will 

Although no quantitative assessments have been re¬ 
ported, it is believed that the decomposers (bacteria, acti- 
nomycetes, and fungi) play the most important role in wet¬ 
lands relative to the removal of organic matter by way of 
mineralization and gasification. They are associated with 
all habitats listed. They are also responsible for the syn¬ 
thesis of biomass and the production of organic metabolic 
end products that may leach into the water column. These 
organisms are often classified as aerobic, facultative, or 
anaerobic, depending on the type of reactions that they 
perform. They may also be classified with respect to their 
source of energy and the type of chemical compound that 
they use as a source of carbon. 

The primary producers generate organic residues within 
the wetland and add dissolved oxygen to the system. They 
also play an important role in the removal and recycling of 
nutrients in the wetland. The macrophytes will also pre¬ 
vent incoming radiation from entering the water column. 
This shading will affect the growth of plant periphyton, phy¬ 
toplankton, and submerged macrophytes, will interfere with 
surface mass transfer of oxygen, and will moderate wet¬ 
land water temperatures. These rooted plants provide sig¬ 
nificant submerged surfaces for growth of periphyton. The 
submerged surface area provided by selected FWS veg¬ 
etation varies from 2.2 to 6.5 m 2 /m 2 for a wetland 0.5 m 
deep, depending on plant species and coverage (EPA, 
1999). 

Oxygen Transfer to FWS Wetlands 

Oxygen is a critical element in the biochemical transfor¬ 
mation of organic matter (as well as other compounds to 
be described later). As briefly discussed previously, bio¬ 
chemical reactions that use dissolved oxygen (DO) are 
efficient processes and yield mineralized end products. An 
aerobic environment is highly desirable in wastewater treat¬ 
ment systems in which the target is effective removal of 
BOD. There are three possible sources of DO in wetland 
systems—surface aeration, photosynthesis, and plant oxy¬ 
gen transfer. 

When a gas is dissolved in water, the process is gener¬ 
ally treated as a mass transfer occurring over four steps 
(two-film theory). It presumes a thin but finite gaseous and 
liquid film at the air-water interface. The gas must pass 
through the bulk gas phase, then through the gas film, to 
the liquid film, and then into the bulk liquid. For oxygen, 
the liquid film is the rate-limiting step and controls the rate 
of mass transfer, although for a quiescent water body, the 
bulk liquid transport may control the process. By use of 
Fick’s Law, the diffusional process can be modeled pro- 


38 



ducing the commonly used oxygen mass transfer relation¬ 
ship: 

dC/dt = K L a (C s -C) ( 3 - 1 ) 

Where C = concentration of oxygen in water (M/L 3 ) 
t = time (T) 

K L a = overall mass transfer coefficient for oxy¬ 
gen in water (1/T) 

C s = steady-state oxygen saturation concentra¬ 
tion in water (M/L 3 ) 

C o = the dissolved oxygen concentration in the 
bulk water (M/L 3 ) 

This equation indicates that the rate of oxygenation is 
dependent on the overall mass transfer coefficient, the 
steady-state DO saturation concentration for oxygen in 
water, and the bulk water DO. Generic symbols for mass 
(M), time (T), and length (L) are employed for units. The 
value of K L a is a function of the physical characteristics of 
the wetland. For typical FWS systems, isotropic turbulence 
would occur whereby there is neither a significant velocity 
gradient nor shearing stress. For this system, the value of 
K L a would be a direct function of wetland velocity and the 
oxygen molecular diffusion coefficient and an indirect func¬ 
tion of system depth (O’Connor and Dobbins, 1958) ac¬ 
cording to the following equation: 

K 2 = [D l U] 1/2 / H 3/2 (3-2) 

Where K, = surface reaeration coefficient (1/T) 

D l = oxygen molecular diffusion coefficient (L 2 /T) 
U = velocity of flow (L/T) 

H = depth (L) 

An estimate of surface aeration in an open water zone 
of a FWS system can be made assuming a typical wet¬ 
land water velocity of 30 m/d and a depth of 0.3 m. At 
20°C, the mass transfer coefficient would be approximately 
0.43/d. Assuming a bulk wetland water DO of 0 mg/L, the 
mass transfer of oxygen would be about 3.9 mg/L/d or 1.2 
g/m 2 /d. For more realistic values of the water DO, the trans¬ 
fer would range from 0.5 to 0.9 g/m 2 /d. These numbers 
compare favorably with values reported by the TVA, which 
range from 0.50 to 1.0 g/m 2 /d (Watson et al., 1987). These 
open water-zone values will be higher than would be ob¬ 
served in wetlands with plant cover because plant debris, 
floating plants, and emergent plant surfaces impede mass 
transfer across the surface. In the open zones, the bulk 
water DO (which might approach 0 mg/L) would not be the 
same as the DO near the water surface (which might ap¬ 
proach saturation during the daylight hours). See Figure 
3-5 for a submergent plant (open water) zone, which illus¬ 
trates the inaccuracy that would result in assuming a DO 
of 0 mg/L in the bulk water column. The actual DO values 
in Figure 3-5 are from polishing FWS applications and are 
therefore higher than would be expected in a treatment 
FWS system. Actually, a value near zero DO is normal in 
fully vegetated or emergent zones (Figure 3-5b) of these 
latter systems. 


Oxygen will also be transferred to the wetland by virtue 
of photosynthesis carried out by phytoplankton, periphy¬ 
ton, and submerged plants. The general relationship for 
photosynthesis of green plants implies that approximately 
2.5 g of oxygen would be evolved per g of carbon fixed as 
cell mass. In the absence of sunlight energy, oxygen would 
be consumed by these plants in respiration. Presuming 
highly productive conditions, about 1.0 g C/m 2 /d may be 
produced during daylight hours resulting in a generation 
of 2.5 g 0 2 /m 2 /d (Lewin, 1962). One may deduce a some¬ 
what larger value presuming typical net primary produc¬ 
tion of a wetland to be about 4.0 g total biomass/m 2 /d 
(Mitsch and Gosselink, 1993) and about 1.0 g 0 2 /g net 
biomass produced by photosynthesis. These numbers 
imply that in open areas where sufficient photosynthesis 
might occur, oxygen should be available for aerobic oxi¬ 
dation within the water column, at least during part of the 
day. Active plant respiration during the evening may com¬ 
pletely negate the oxygenation estimated during daylight 
hours, however. The DO concentrations have been docu¬ 
mented for both emergent and submergent plant zones in 
the Areata tertiary/polishing/enhancement marsh system 
(Figure 3-5). Clearly, the submergent plant zone provided 
more DO by virtue of greater photosynthesis and surface 
aeration. This figure also clearly demonstrates that the 
bulk water column is not mixed with respect to DO, being 
highest at the surface where atmospheric reaeration and 
photosynthesis are predominant. Rose and Crumpton 
(1996) have demonstrated the effects of emergent mac¬ 
rophyte stands and open water in a prairie wetland. They 
found that the emergent sites had low DO concentrations 
and were almost always anoxic, whereas sites at the edge 
of the stand had higher DO concentrations, with signifi¬ 
cant increases in DO during the daylight hours. The open 
water areas consistently exhibited higher values of DO 
with diurnal changes up to 10 mg/L. These results are 
consistent with the presumption that emergent stands pro¬ 
vided a heavy canopy cover, along with small floating 
plants and plant litter that obscured light penetration and 
subsequent photsynthesis and also affected surface aera¬ 
tion. The plant litter stands at the margin were open and 
allowed light penetration, encouraging photosynthesis by 
periphyton, phytoplankton, and submerged plants. Open 
areas allowed even more opportunity for photosynthesis 
as well as surface aeration. 

The role of rooted, emergent plants on oxygen transfer 
to the wetland system is subject to controversy. Because 
these wetland plants are typically rooted in soils that are 
anaerobic, they have evolved special airways to allow the 
efficient movement of atmospheric oxygen to the root sys¬ 
tem. Dead and broken plant shoots may also allow for trans¬ 
port of some oxygen to the root zone. There is a sufficient 
body of evidence to show that significant amounts of oxy¬ 
gen are transported down these passages and that other 
gases, such as carbon dioxide, hydrogen, and methane, 
may pass upward through these same channels. Gas trans¬ 
fer is the result of humidity-, thermally-, and/or Venturi-in¬ 
duced convective flow as well as diffusion. Gas exchange 
through the root surface is by diffusion. Although photo- 


39 




a) Vertical distribution of DO in a submergent 
plant zone of the Areata Enhancement Marsh 


O) 

g 

O 

o 



Gearheart-6 
Gearheart-4 
Gearheart-2 


middle 


bottom 


b) Vertical distribution of DO in an emergent 
plant zone of the Areata Enhancement Marsh 


Figure 3-5. Dissolved oxygen distribution in emergent and submergent zones of a tertiary FWS (EPA, 1999) 


synthesis does not seem to enhance the concentration of 
oxygen in the shoots, sunlight can cause convection to 
increase because light amplifies the degree of stomatal 
openings in the leaves, and higher temperatures cause 
steeper diffusion gradients for gases (Armstrong and 
Armstrong, 1990). A large proportion of the oxygen enter¬ 
ing the plant is vented to the atmosphere instead of being 
used in plant respiration or effluxed to the root system. 

A number of studies have been made on the transport of 
oxygen by plants. Only one field study has been reported 
on an operating wetland (Brix and Schierup, 1990). They 
found that for Phragmites in both FWS and VSB systems, 
the oxygen transported almost exactly balanced respira¬ 
tion by the plants. The range of values reported by other 
investigators is 0 to 28.6 g 0 2 /m 2 /d, whereas rates of 0 to 3 
g 0 2 /m 2 /d were found in 15 of the 23 studies. Kadlec and 
Knight (1996) pointed out that studies to date infer that 
oxygen transfer measurements made from BOD and am¬ 
monium losses are not accurate. At present, it seems rea¬ 
sonable to presume that plant oxygen transfer is not an 
important source of oxygen in most wetland systems. 

The DO concentration found in FWS systems is depen¬ 
dent on the rate of oxygen transfer and the rate of oxygen 
uptake. If one equals the other, system DO will remain 
constant. If respiration exceeds transfer, DO values will 
fall to zero and anaerobic conditions will prevail. The prin¬ 
ciple consumers of oxygen (respiration) in wetlands include 
microorganisms that consume oxygen during normal ex¬ 
ogenous and endogenous respiration and plants that carry 
out respiration when sunlight energy is unavailable as an 
energy source. Sources of oxygen requirements will in¬ 
clude those from influent organic matter, stored organic 
matter in living biomass (endogenous respiration), dead 


plant litter at the surface and bottom of the wetland, dead 
periphyton and plankton suspended in the water column 
or residing at the wetland surface or within the benthal 
deposits, and influent ammonium nitrogen. One factor that 
should be considered in wetland design is the determina¬ 
tion of an oxygen balance. A mass balance on oxygen is 
difficult to perform in FWS systems owing to the highly 
complex and dynamic characteristics of the system. An 
estimate of influent oxygen demand (carbonaceous and 
nitrogenous) is possible, but uptake due to plant respira¬ 
tion and decomposition of plant carbon is more difficult to 
predict (Table 3-6). The sources of oxygen are also diffi¬ 
cult to quantify as discussed previously. Total coverage of 
wetlands by emergent plants results in little reliable oxy¬ 
genation (some oxygen will likely be produced by periphy¬ 
ton near the water surface, but quantification of this is un¬ 
reliable at the present time). Use of open areas to pro¬ 
mote photosynthesis by submerged plant and plankton 
appears to be a logical approach, but again, quantitative 
estimates of transfer are difficult to assess based on cur¬ 
rent data. It should be emphasized that DO is not a steady 
value in wetlands but will vary in a diurnal fashion with 
photosynthesis. It is not unrealistic to presume that wide 
fluctuations in DO will occur, especially in active systems. 
As will be described later, the DO concentration will also 
change along the wetland length as demands and sup¬ 
plies change. 

FWS Biological Fleactions 

Influent particulate organic matter may be entrapped 
within biofilms attached to emergent plant surfaces or ac¬ 
cumulated on the wetland floor within the plant litter and 
sediments (Figure 3-4). Experience suggests that much of 
this material is accumulated very close to the influent struc- 


40 





























Table 3-6. Wetland Oxygen Sources and Sinks 


Process 

Source 

FWS 

Sink 

VSB 

Source Sink 

Reaeration 

+ o 


+ 

Photosynthesis 

+ 



Plant 0 2 Transpiration 

+ 


+ 

Influent BOD ult ; (oxid./endog.) 


+ (t) 

+ (t) 

Influent NH. 


+ (+) 

+ (t) 

Plant Respiration 


+ 

+ (t) 

Plant Decomposition 


+ 

+ (t) 


(*) may be estimated in some instances 
(t) May be calculated for wetland system 


ture. In addition, particulate organic matter deriving from 
dead plant litter accumulates on the floor of the wetland as 
well as in surface mats. The quantity of this material is 
dependent on the species of plants growing in the area 
and their coverage. Emergent plant communities have 
higher potential production rates than submerged commu¬ 
nities (Wetzel, 1983). The accumulated organic debris at 
the wetland floor degrades at different rates depending on 
the composition of the organic matter. Influent particulate 
organic matter from primary settling or septic tank efflu¬ 
ents is easily degradable in most environments. Algal cells 
from pond effluents are biologically less available. Emer¬ 
gent macrophytes produce much more structural material 
(lignin, cellulose, and hemicellulose) than do submerged 
and floating leafed plants, and this material degrades rela¬ 
tively slowly (Godschalk and Wetzel, 1978). 

Much of the particulate organic matter would be hydro¬ 
lyzed, producing lower molecular weight organic com¬ 
pounds that are more soluble in water, which leach back 
into the water column and contribute to the soluble BOD 
downstream. In the presence of oxygen, these compounds 
would be oxidized by microbes to C0 2 , oxidized forms of 
nitrogen and sulfur, and water. Under anaerobic conditions, 
these compounds may be converted to low molecular 
weight organic acids and alcohols. Under strict anaerobic 
conditions, methanogenesis may occur whereby these 
compounds are converted to gaseous end products of CH 4 , 
C0 2 , and H 2 . In the presence of sulfates, sulfur-reducing 
microbes will convert these low molecular weight organic 
compounds to C0 2 and sulfide. Either of these anaerobic 
reactions will essentially remove organic matter from the 
system. As the degradability of the material decreases, 
the decomposition rates slow and the nature of the meta¬ 
bolic end products change. It has been suggested that 
soluble organic matter has a half-life of about three days 
while organic sediment may exhibit a half-life on the order 
of four months (EPA, 1999). The rates of degradation are 
also temperature dependent. Thus sediment organic mat¬ 
ter may accumulate during the colder months and be more 
rapidly degraded in the spring when water temperatures 
rise. This increase in degradation will result in an increase 
in soluble organic matter released to the water column and 
a concomitant increase in oxygen demand, a phenomenon 
that has also been observed in facultative ponds for nearly 
a century. 


The result of these metabolic activities is that (1) organic 
matter concentrations (and oxygen demand) may increase 
downstream as particulate organic material is solubilized, 
(2) the effect of temperature on observed organic matter 
removal may not be as significant in wetlands as would be 
predicted by typical temperature correction relationships 
for biochemical reactions, and (3) the organic matter re¬ 
sidual downstream is a combination of recalcitrant organic 
matter in the influent, likely very small, soluble organic 
compounds released from plant decomposition and par¬ 
ticulate organic matter released from dead plant and mi¬ 
crobial materials. 

The source of the soluble fraction of organic matter in 
the wetland is, as has been stated, influent to the system 
and soluble material released from the decomposition of 
influent particulate matter and dead plant and microbial 
tissue. This soluble material is most likely sorbed onto plant 
surface biofilms, although a fraction may be sorbed onto 
biomass suspended in the water column, and still another 
fraction will diffuse to the debris at the wetland floor or 
surface. The sorbed organic matter will be metabolized by 
organisms associated with the biofilms. The metabolic 
pathway and the end products of this metabolism will be 
dependent on the presence or absence of oxygen. As pre¬ 
viously described, areas of the wetland populated with 
dense emergent macrophytes can supply only a small frac¬ 
tion of the oxygen necessary to satisfy demand. Typical 
wetland profiles in these areas suggest that most of the 
water column is anoxic, as are the sediments below (small 
microsites containing oxygen may be found adjacent to 
active plant roots). The degree of anoxia would be depen¬ 
dent on the organic and nutrient load to the wetland area. 
Open areas of wetland (containing submerged plants) and 
the margins adjacent to emergent macrophyte coverage 
do demonstrate aerobic conditions throughout the wetland 
depth (again, dependent on organic and nutrient load and 
type of vegetation). 

As described previously, the mechanisms that regulate 
dissolved organic matter removal in wetlands include bio¬ 
degradation, sorption, and photolysis. Different operative 
mechanisms may act on different types of organic matter. 
As a result, fundamental mechanisms should be consid¬ 
ered in wetland designs and operation to enhance removal 
processes. Wetlands receiving municipal wastewater pond 
effluents may, for example, produce a net increase in dis¬ 
solved organic matter (Barber et al., 1999). Details of de¬ 
sign strategies for FWS systems based on these observa¬ 
tions are found in Chapter 4. 

3.3.3 Organic Matter in Vegetated 
Submerged Beds 

Vegetative submerged beds act as fixed-film bioreactors. 
As described in section 3.2.3, the actual role of plants in 
these beds is controversial. Some research (Gersberg et 
al., 1986) has claimed effective results with selected spe¬ 
cies, but these claims have not been substantiated by oth¬ 
ers. The presence of a root structure would provide addi- 


41 







tional surface for biofilm attachment. Macrophytes may also 
contribute some oxygen to the granular bed as previously 
described. Based on that review, rates of oxygen trans¬ 
port by macrophytes range from 0 to 3 g 0 2 /m 2 /d. How¬ 
ever, it has been found that root penetration in the bed is 
only partial, and there is a significant amount of flow under 
the root zone. Furthermore, plant oxygen transfer would 
be unreliable for a significant portion of the year due to 
plant senescence. 

Particulate organic matter is removed in VSB systems 
by mechanisms similar to suspended solids separation in 
horizontal gravel beds with the same media size. The sepa¬ 
rated solids from the influent wastewater and plant litter 
would undergo decomposition much the same way as oc¬ 
curs in FWS systems. Hydrolysis will generate soluble 
compounds. Those compounds and soluble organic mat¬ 
ter from the influent to the system or cycled from solids 
decomposition will most likely sorb to biofilm surfaces at¬ 
tached to the media, plant roots, and plant litter accumu¬ 
lated at the bed surface or within the interstices of the 
media. Oxygen sources to the VSB would be limited to 
some small amount of surface aeration and plant-medi¬ 
ated transport. The thatch that accumulates at the bed 
surface would inhibit or at least slow down surface trans¬ 
port. It is possible that some aerobic metabolism would 
occur in these beds, but the predominant biological mecha¬ 
nism is likely to be facultative/anaerobic. Typical values of 
DO in VSB systems are very low (<0.1 mg/L). In VSB sys¬ 
tems where ORP has been measured, values were typi¬ 
cally quite low, indicating strong reducing conditions 
(Lienard, 1987). Thus the predominant metabolic pathways 
are most likely anaerobic. As described previously, the 
anaerobic pathways leading to BOD removal from the sys¬ 
tem would be methanogenesis, sulfate reduction, or deni¬ 
trification, all yielding gaseous end products. These reac¬ 
tions are temperature dependent and therefore are likely 
to slow down or cease in winter months. As discussed pre¬ 
viously, the processes will resume as the water warms up, 
and high releases of gaseous end products and soluble 
organic matter may occur. Some clogging of the bed may 
occur due to accumulation of slowly degradable and re¬ 
calcitrant solids. Low-loaded systems may exhibit some 
aerobic reactions, especially near the effluent end of the 
process. Residual effluent BOD from VSB systems is likely 
to be somewhat more consistent than that from FWS sys¬ 
tems because of the presence of less plant matter in the 
water column. 

3.4 Mechanisms of Nitrogen Separations 
and Transformations 

3.4.1 Description and Measurement 

In waters and wastewater, the forms of nitrogen of great¬ 
est interest are, in order of decreasing oxidation state, ni¬ 
trate, nitrite, ammonia, and organic nitrogen. All nitrogen 
forms are reported in wastewater as nitrogen, N. All of these 
forms, including nitrogen gas (N 2 ), are biochemically 
interconvertible and are components of the nitrogen cycle. 
Analytically, organic nitrogen and ammonia can be deter¬ 


mined together, and are termed “Total Kjeldahl nitrogen” 
(TKN). Organic nitrogen in wastewater includes proteins, 
peptides, nucleic acids, and urea. Organic nitrogen may 
be found in both soluble and particulate forms. The other 
nitrogen species are water soluble. Ammonia nitrogen may 
be found in the un-ionized form, NH 3 , or the ionized form, 
NH 4 + , depending on water temperature and pH. The ion¬ 
ized form is predominant in wetlands. At 25°C and a pH of 
7.0, the % un-ionized ammonia is approximately 0.6%. 

The discharge of nitrogen to receiving surface and 
ground water sources is of concern for a number of rea¬ 
sons. Excessive accumulation of nitrogen in surface wa¬ 
ters can lead to ecological imbalances that may cause 
overgrowth of plants and animals, leading to water quality 
degradation (eutrophication). High concentration of the un¬ 
ionized ammonia species are toxic to fish and other aquatic 
life. Nitrate and nitrite nitrogen constitute a public health 
concern, primarily related to methemoglobinemia and car¬ 
cinogenesis. Ammonia nitrogen may deplete dissolved 
oxygen in natural waters by way of microbial nitrification 
reactions. As a result, discharge permits may be written to 
control any or all species of nitrogen. Most commonly, 
ammonia or total nitrogen are the target pollutants speci¬ 
fied depending on the receiving stream. Typical concen¬ 
trations of the nitrogen species found in primary, septic 
tank, and treatment pond effluents are shown in Table 3-1. 
It should be noted that whereas primary and septic tank 
effluents would contain organic nitrogen and ammonia, 
treatment ponds may contain either reduced or oxidized 
forms of nitrogen depending on loading and season of the 
year. Organic nitrogen in the latter systems would be pri¬ 
marily associated with algal cells. It is important to note 
that when evaluating the performance of wetlands relative 
to nitrogen, both total nitrogen and the species of nitrogen 
are important. Mass balances must be conducted on total 
nitrogen species, not on just one or two forms, to generate 
meaningful data. 

3.4.2 Nitrogen in Free Water Surface 
Systems 

3.4.2.1 Physical Separation of Nitrogen 
Species 

There are a number of separation processes that will 
affect nitrogen species in wetlands. Nitrogen associated 
with suspended solids (organic nitrogen) may be removed 
by many of the processes described earlier for the removal 
of TSS including flocculation, sedimentation, filtration, and 
interception processes (Figure 3-6). Sorption of both par¬ 
ticulate and soluble organic nitrogen may occur on biofilms 
associated with emergent macrophytes, plant litter, or other 
detritus at the FWS surface or on the bottom. Ion exchange 
of ammonium (NH 4 + ) by clay minerals in the wetland soils 
may play a role in nitrogen separation if this species either 
diffuses into the soil layer or is biologically produced by 
the ammonification of organic nitrogen solids located in 
the benthal layer. The exchange capacity of the clay min¬ 
erals would be important in this assessment. It must be 
emphasized that the exchange mineral would have limited 


42 




Sediment 


PON - Particulate Organic Nitrogen 
DON - Dissolved Organic Nitrogen 


Figure 3-6. Nitrogen transformations in FWS wetlands 


long-term capacity unless regeneration by chemical or bio¬ 
chemical action takes place. It is not likely that ion exchange 
would play an important or dependable role in nitrogen 
removal after a start-up period in most wetland systems. 
Furthermore, the native soil layer is buried under detritus 
and plant litter within a period of time, thereby isolating 
these clay minerals from the wetland system. Ammonia 
(NH 3 ) gas may also be removed from the system by strip¬ 
ping. As discussed previously, the quantity of un-ionized 
ammonia at neutral pH values is low, but during active 
photosynthesis in open water zones, pH values may rise 
to values as high as 8.0 to 8.5 depending on water alkalin¬ 
ity. At that pH, the fraction of NH 3 (NH 3 /NH 4 + ) may increase 
to 20 to 25% at 20°C. If surface turbulence is high due to 
wind action, significant losses of nitrogen may occur in 
these open water areas if ammonia concentrations are 
high. Few, if any, well-controlled studies have been con¬ 
ducted to examine the importance of volatilization losses 
in FWS systems with open areas. 

3.4.2.2 Biologically Mediated Transformations 
of Nitrogen Species 

Ammonification 

Almost half of the municipal wastewater nitrogen con¬ 
tent, as received at the treatment facility, is in the organic 
nitrogen form. The rest has usually already been converted 
to ammonium-nitrogen (EPA, 1993) in the sewer. The bio¬ 
logical transformation of organically combined nitrogen to 
ammonium nitrogen during organic matter degradation is 


referred to as ammonification, hydrolysis, or mineraliza¬ 
tion. The process occurs under aerobic and anaerobic 
conditions, but has been described as slower for the latter 
by Mitsch and Gosselink (1993). Other environmental en¬ 
gineering literature, however, does not support any differ¬ 
ence in rates under varying oxygen states because of their 
primary dependence on enzymatic pathways. The rate of 
this process is primarily dependent on pH and tempera¬ 
ture, increasing with increased temperatures. Municipal 
wastewaters have been demonstrated to be fully hydro¬ 
lyzed in 19 hours at temperatures of 11° to 14°C (Bayley 
et al., 1973). Once the ammonium is formed, it can be 
absorbed by plants through their root systems, immobi¬ 
lized by ion exchange in the sediments, solubilized and 
returned to the water column, volatilized as gaseous am¬ 
monia, anaerobically converted back to organic matter by 
microbes, absorbed by phytoplankton/floating aquatic 
macrophytes in the water column, or aerobically nitrified 
by aerobic microorganisms. 

Nitrification 

In the presence of dissolved oxygen, microbes in the 
water column or within the biofilms may convert ammo¬ 
nium to nitrite and nitrate nitrogen in a two-step process. 
In this process about 4.3g of 0 2 are required per g ammo¬ 
nium nitrogen oxidized to nitrate and 7.14 g of alkalinity as 
CaC0 3 are consumed. The process is temperature and 
pH dependent (EPA, 1993). The reaction may take place 
in an aerobic water column by suspended bacteria and 


43 












































within any aerobic biofilms. Nitrate is not immobilized by 
soil minerals and remains in the water column or pore water 
of the sediments. It may be absorbed by plants or microbes 
in assimilatory nitrate reduction (converted to biomass via 
ammonium) or may undergo dissimilatory nitrogenous ox¬ 
ide reduction (nitrate reduction pathways referred to as 
denitrification). 

Denitrification 

Dissimilatory nitrate reduction or denitrification is car¬ 
ried out by microorganisms under anaerobic (anoxic) con¬ 
ditions, with nitrate as the terminal electron acceptor and 
organic carbon as the electron donor (EPA, 1993). That is 
to say, the reaction occurs in the absence of oxygen and 
requires an organic carbon source. The products of deni¬ 
trification are N 2 and N 2 0 gases that will readily exit the 
wetlands. The denitrification reaction occurs primarily in 
the wetland sediments and in the periphyton films in the 
water column below fully vegetated growth where DO is 
low and available carbon is high. The minimum carbon to 
nitrate-nitrogen ratio for denitrification would be about 1 g 
C/g N0 3 -N. Decomposing wetland plants and plant root 
exudates are potential sources of biodegradable organic 
carbon for this purpose. These sources are most readily 
available at the beginning of senescence. Organic carbon 
will be consumed (satisfying some oxygen demand: ap¬ 
proximately 2.86 g 0 2 per g nitrate nitrogen reduced) and 
alkalinity is produced (approximately 3.0 g CaC0 3 alkalin¬ 
ity per g nitrate nitrogen reduced). The process is tem¬ 
perature and pH dependent. Denitrification in the sediments 
may supply N 2 for fixation by bacteria and for plant uptake 
in the root zone (nitrogen fixation) if the system is nitro¬ 
gen-poor. The remainder will remain in equilibrium with N 2 
in the water column. The nitrogen gas in the water phase 
may be available for nitrogen fixation by some periphyton 
and phytoplankton. There will also be an air-water inter¬ 
change of N 2 . The losses to the air represent nitrogen 
losses from the system, and these are greater in open water 
than in fully vegetated zones. 

Nitrogen Fixation 

Nitrogen gas may be converted to organic nitrogen by 
way of selected organisms that contain the enzyme nitro- 
genase. The reaction may be carried out aerobically or 
anaerobically by bacteria and blue-green algae. Nitrogen 
fixation occurs in the overlying water in FWS open water 
zones, in the sediment, in the oxidized rhizosphere of the 
plants, and on the leaf and stem surfaces of plants (Reddy 
and Graetz, 1988). It may be a significant source of nitro¬ 
gen in natural wetlands but is not important in systems 
treating wastewater in which nitrogen is plentiful. 

Plant Uptake (Assimilation) 

Wetland plants will assimilate nitrogen as an important 
part of their metabolism. Inorganic nitrogen forms are re¬ 
duced by the plant to organic nitrogen compounds used 
for plant structure. During the growing season, there is a 


high rate of uptake of nitrogen by emergent and submerged 
vegetation from the water and sediments. Increased im¬ 
mobilization of nutrients by microbes and uptake by algae 
and epiphytes also lead to a retention of inorganic forms 
of nitrogen in the wetland. Estimates of net annual nitro¬ 
gen uptake by emergent wetland plant species vary from 
0.5 to 3.3 gN/m 2 /yr (Burgoon et al., 1991). Reeds and bul¬ 
rushes are at the lower end of this range, whereas cattails 
are at the higher end. Estimates for epiphytes and microbes 
in wetland systems have not been found. During the ac¬ 
tive growth period, a significant amount of the total plant 
nitrogen is in the stems and leaves above the sediments. 
During senescence, the nitrogen translocates back to the 
roots and rhizomes for storage. However, a substantial 
portion of the nitrogen is lost to the water column through 
litter fall and subsequent leaching. This generally leads to 
a net export of nitrogen in the fall and early spring. The 
extent of recycling of nitrogen within the wetland is depen¬ 
dent on nitrogen loading to the system. Treatment wetland 
systems are considered as eutrophic wetlands with ex¬ 
cessive nutrient levels. As a result, intrasystem cycling is 
less important to the treatment process than it would be if 
the hydrologic regime were more varied (Twinch and 
Ashton, 1983). 

3.4.2.3 Nitrogen in Free Water Surface 
Wetlands 

Figure 3-6 illustrates the highly complex series of reac¬ 
tions for nitrogen in FWS systems. Particulate organic ni¬ 
trogen entering the wetland as wastewater influent or pro¬ 
duced in the wetland by plants is separated. It may be 
found associated with biofilms attached to plant structures 
in the water column, the wetland sediment, or in floating 
litter and detritus. The biodegradable compounds will be 
ammonified by aerobic or anaerobic organisms associated 
with the biofilms and sediment surfaces. The recalcitrant 
organic nitrogen will accumulate and eventually become a 
part of the deep sediments. 

Ammonium released from the particulate organic nitro¬ 
gen in the sediments is available to the emergent and sub¬ 
merged macrophytes as an important nutrient. Uptake 
occurs during the growing season, which increases the 
concentration gradient and the release of more ammonium. 
Excess ammonia may remain in reserve in the sediment 
and leach from the sediment into the water column, where 
it may undergo biological oxidation (nitrification) under 
aerobic conditions. Release of ammonium into the water 
column in the fall and early spring is not an unusual phe¬ 
nomenon in wetlands. The leached ammonium may be 
taken up by epiphytes or plankton found in the water col¬ 
umn or be attached to emergent and floating plants. Nitri¬ 
fication of ammonium requires DO and is therefore limited 
to areas of the wetland where oxygen is available. There 
may be some nitrification occurring adjacent to plant rhi¬ 
zomes where oxygen leaks from the plant. The relative 
importance of this pathway in wetlands is minor in treat¬ 
ment wetlands because most sediments below emergent 
canopies are anaerobic. Little nitrification would be ex¬ 
pected in these regions of the wetland. In the water col- 


44 



umn near the surface in open areas, oxygenation may be 
sufficient to ensure significant nitrification. The important 
variable in sizing an open water zone of a FWS system for 
nitrification is organic plus nitrogenous loading. More pre¬ 
cisely, it is the oxygen demand loading (carbonaceous plus 
nitrogenous oxygen demand [CBOD + NOD]) to each wet¬ 
land zone that will dictate whether oxygen is present. Typi¬ 
cally, nitrification will not be initiated until a majority of the 
organic compounds have been removed. Thus, nitrifica¬ 
tion in the water column would not be expected in the ini¬ 
tial settling zone, but could occur in subsequent open wa¬ 
ter/aerobic zones. 

The nitrate produced by nitrification or introduced with 
the system influent (e.g., from oxidation ponds achieving 
nitrification) may be taken up by periphyton or plankton. 
Small amounts produced within any aerobic sediments 
would be taken up by plant roots or may diffuse upward 
into the water phase. Under anaerobic conditions and in 
the presence of organic matter, microbes associated with 
attached biofilms or suspended in the water column may 
convert nitrate to nitrogen gases (NO N 2 ) via denitrifica¬ 
tion. Some nitrate will also diffuse into tne sediments where 
it is available for plant uptake or can be denitrified as well. 

Moving downstream from the wetland influent, the reac¬ 
tions of nitrogen could be expected to occur sequentially 
such that total nitrogen levels should drop in the settling 
zone (owing to separation of organic nitrogen), followed 
by ammonia release, nitrification, and denitrification. Plants 
will attenuate this sequence by releases and uptake 
throughout the annual growth/senescence cycle. Systems 
loaded at a level such that oxygen demand exceeds oxy¬ 
gen supply will not exhibit significant nitrification. Seasonal 
change as well as influent variability will greatly impact 
system performance. Highly nitrified influent from pretreat¬ 
ment systems may provide excellent nitrogen removal 
during warm seasons when sufficient organic matter is 
available (primarily from plant decomposition) for denitrifi¬ 
cation. Nitrogen release may be significant in fall and early 
spring seasons during plant senescence and death. The 
background nitrogen values found in Table 3-5 reflect con¬ 
tributions by internal recycling of nitrogen within the sys¬ 
tem. 

In open water zones of FWS systems, elevated pH and 
water temperature may enhance NH3-N volatization to the 
degree that it becomes a significant removal mechanism. 
This mechanism has been shown to reach levels as high 
as 50% or more under optimum conditions in stabilization 
ponds, but FWS open water zones are smaller in size, 
therefore minimizing this pathway under normal conditions 
(EPA, 1983). 

Wastewater discharge permits are normally written so 
as to limit effluent ammonia concentrations (either sea¬ 
sonally or all year) or total nitrogen concentrations. For 
ammonia removal, the processes that will achieve effec¬ 
tive removal include plant uptake, nitrification, volatiliza¬ 
tion, and ion exchange. The latter two typically have only 
minor impact in most FWS systems. Plant uptake is sea¬ 


sonal and requires harvesting prior to plant senescence. 
Fortunately, seasonal nitrogen uptakes may parallel am¬ 
monia restrictions in those cases in which these restric¬ 
tions are based on summer oxygen demands and/or cer¬ 
tain game fish maximization. Generally, the designer must 
be concerned with achieving reliable ammonia removal by 
means of nitrification. This may be achieved at low load¬ 
ing (oxygen demand) with sequencing closed and open 
wetland areas (See Chapter 4). It should be noted that 
nitrification is temperature dependent, so that rates will sig¬ 
nificantly slow in winter months, especially in colder cli¬ 
mates. 

Nitrogen removal may be achieved by plant uptake/har¬ 
vesting, nitrification/denitrification, volatilization, and ion 
exchange. Again, the latter two are considered to be of 
minor consequence in most FWS systems. Plant uptake 
and harvesting requires careful system management and 
can be costly. The requirement for denitrification requires 
nitrification as explained previously plus adequate decay¬ 
ing plant organic carbon in a region free of dissolved oxy¬ 
gen. Sequential designs are also best to achieve this goal 
(See Chapter 4). Note that temperature affects both nitrifi¬ 
cation and denitrification so that rates can be significantly 
reduced during the colder months, which may control de¬ 
sign requirements. 

3.4.3 Nitrogen in Vegetated Submerged 
Beds 

As described in section 3.3.3, VSB systems incorporate 
anaerobic fixed-film biological reactions. Organic nitrogen 
trapped within the bed will undergo ammonification. The 
released ammonia may be available for plant uptake de¬ 
pending on the location of plant roots. Flow below the plant 
roots will carry ammonium downstream. Plant uptake (0.03 
to 0.3 g/m 2 /d) of nitrogen is low compared to typical nitro¬ 
gen loading to VSB systems. As described previously, de¬ 
pendence on plant uptake for nitrogen removal requires 
harvesting and is not effective during plant senescence 
and death. 

Oxygen sources in VSB systems are negligible, and it is 
most likely there will be insufficient oxygen to promote re¬ 
liable nitrification in all but the most lowly loaded systems. 
Any nitrification occurring may be found in the root zone 
adjacent to rhizomes or near the bed surface where some 
surface oxygen transfer might occur. If nitrification occurs, 
it would occur downstream where oxygen demand is low¬ 
est. 

Conventional VSB systems would seem to be well suited 
for denitrification of nitrified influents. These beds are 
anaerobic. However, they require a supply of organic car¬ 
bon from decomposing plant residue entrapped within the 
bed or aerobically decomposed products of plant biomass 
at the bed surface, which may also leach into the anaero¬ 
bic zones during rainfall events. The supply of carbon is 
seasonal, however, since it would be highest after plant 
senescence. Low temperature will slow the process dur¬ 
ing the winter months. 


45 




From this discussion, it is clear that conventional VSB 
systems do not represent reliable, cost-effective systems 
for ammonia removal. To improve, the loading to the sys¬ 
tem would necessarily have to be low. Nitrogen removal 
from well-nitrified influents to the system may be possible, 
but seasonal availability of carbon may create carbon limi¬ 
tations that should be considered in the design (See Chap¬ 
ter 5). 

3.5 Mechanisms of Phosphorus 
Separations and Transformations 

3.5.1 Description and Measurement 

Phosphorus occurs in natural waters and wastewater 
primarily as phosphates. They are classified as orthophos¬ 
phates, condensed (pyro-, meta-, and poly-) phosphates, 
and organically bound phosphates. They may be in solu¬ 
tion or particulate form. Organic phosphates are formed 
primarily by biological processes and are found in raw 
wastewater as food residues and body wastes and in 
treated wastewater as living or nonliving biota (e.g., algae 
and bacteria from treatment ponds). Inorganic phospho¬ 
rus found in wastewater most often comes from various 
forms of personal and commercial cleaning solutions or 
from the treatment of boiler waters. Storm waters carry 
inorganic forms of phosphorus from fertilizers into com¬ 
bined sewers. Classification of phosphorus is based on a 
variety of analytical methods. The typical concentrations 
of phosphates in influent wastewaters to a treatment wet¬ 
land appear in Table 3-1. Modern P concentrations are 
-50% of those shown for septic tank effluent in this table. 
Table 3-3 indicates that the major fraction of these phos¬ 
phate forms are in the colloidal range, and Munch et al. 
(1980) indicated that 80% of the phosphate was split al¬ 
most equally between colloidal and supracolloidal fractions. 

Phosphorus is one of the most important elements in 
ecosystems. It is often the major limiting nutrient in fresh¬ 
water systems. Since there is no important gaseous com¬ 
ponent in the biogeochemical cycle, phosphorus tends to 
move to the sediment sink in natural systems and become 
scarce in the ecosystem. In fact, it is the accretion of min¬ 
eral phosphates and biomass in the sediment that is the 
primary mechanism for phosphorus removal in the wet¬ 
land environment. 

3.5.2 Phosphorus in Free Water Surface 
Systems 

3.5.2.1 Physical/Chemical Separations of 
Phosphorus 

Particulate phosphate may be deposited onto the FWS 
system sediment by sedimentation or entrapped within the 
emergent macrophyte stem matrix and attached (sorbed) 
onto biofilms (Figure 3-7). Soluble phosphate may be 
sorbed onto plant biofilms in the water column, onto biofilms 
in the floating plant litter, or onto the wetland sediments. 
The exchange of soluble phosphate between sediment 
pore water and the overlying water column by diffusion 
and sorption/desorption processes is a major pathway for 
soluble phosphates in wetlands. In the sediment pore wa¬ 


ter, these phosphates may be precipitated as the insoluble 
ferric, calcium, and aluminum phosphates or adsorbed onto 
clay particles, organic peat, and ferric and aluminum ox¬ 
ides and hydroxides. The precipitation as calcium phos¬ 
phates occurs at pH values above 7 and may occur within 
the sediment pore water or in the water column near ac¬ 
tive phytoplankton growth where pH values may rise well 
above 7. The sorption of phosphorus on clays involves 
both the chemical bonding of the negatively charged phos¬ 
phates with positively charged clay and the substitution of 
phosphates for silicates in the clay matrix (Stumm and 
Morgan, 1970). Phosphate can be released (desorbed) 
from the metal complexes depending on the redox poten¬ 
tial of the sediment. Under anoxic conditions, for example, 
the ferric compound is reduced to the more soluble fer¬ 
rous compound and phosphate is released. Phosphates 
may also be released from ferric and aluminum phosphates 
under anoxic conditions by hydrolysis. Phosphate sorbed 
to clays and hydrous oxides may also be resolubilized 
through the exchange of anions. The release of phosphate 
from insoluble salts will also occur if the pH decreases as 
a result of the biological formation of organic acids, nitrates, 
or sulfates. Over time, however, a significant fraction of 
the initially removed phosphate will become bound within 
the sediments and lost to the system. At the start-up of a 
FWS system (possibly for more than one year), the phos¬ 
phorus removal will be abnormally high owing to the initial 
reactions with the soils of the wetland. This removal mecha¬ 
nism is finite and essentially disappears after this period. 

3.5.2.2 Biological Transformations of 
Phosphates 

Dissolved organic phosphate and insoluble inorganic and 
organic phosphate are not usually available to plants until 
transformed to a soluble inorganic form. These transfor¬ 
mations may take place in the water column by way of 
suspended microbes and in the biofilms on the emergent 
plant surfaces and in the sediments. Uptake of phosphates 
by microorganisms, including bacteria, algae, and duck¬ 
weed, acts as a short-term, rapid-cycling mechanism for 
soluble and insoluble forms. Cycling through the growth, 
death, and decomposition process returns most of the 
phosphate back into the water column. Some phosphate 
is lost in the process due to long-term accretion in newly 
formed sediments. Uptake by the macrophytes occurs in 
the sediment pore water by the plant root system. The 
estimate of net annual phosphorus uptake by emergent 
wetland species varies from 1.8 to 18 g P/m 2 /y (Burgoon 
et al., 1991). The cycle of uptake and release is similar to 
that of the microbes, but these reactions operate over a 
longer time scale of months to years. Uptake occurs dur¬ 
ing the growth phase of the plant and release occurs dur¬ 
ing plant senescence and death in the late summer and 
fall, followed by decomposition in the plant litter. Again, 
some phosphate is lost to the system through accretion 
processes within the sediments. 

3.5.2.3 FWS System Reactions—Phosphate 

Sustainable phosphate removal in a FWS system in¬ 
volves the accretion and burial of phosphate within the 


46 




Figure 3-7. Phosphorus cycling in a FWS wetland (adapted from Twinch and Ashton, 1983) 


wetland sediments (Figure 3-7). Phosphate cycling and 
storage involves a complex set of processes with a num¬ 
ber of forms of phosphate. Insoluble organic and inorganic 
phosphates are settled or captured on solid surfaces within 
the water column or in the wetland sediments or floating 
litter. Many of these insoluble forms may be chemically 
and biologically transformed to available forms of phos¬ 
phate for uptake by macrophytes, epiphytes, and floating 
plants. The phosphate taken up by these biological sys¬ 
tems is recycled back into the system and is subsequently 
available for other organisms or may leave the system 
through the water column. The undecomposed portion of 
the biological growths may accumulate and thereby be 
removed from the system as accreted sediment. This ma¬ 
terial along with any recalcitrant phosphate separated from 
the water column and accumulated within the sediment 
represents the total net phosphate removal from the sys¬ 
tem. 

Phosphate removal in FWS systems follows a seasonal 
pattern in most temperate climate conditions. The form of 
phosphate, the type and density of the aquatic plants, the 
phosphate loading rate, and the climate determine the 
pattern and amount of phosphate removed from the wet¬ 
land over any given time period. Aquatic plants serve as 
the seasonal reservoir for phosphate as they take up 
soluble reactive phosphate (SRP) during the growing sea- 
i son. There is a finite amount of SRP that can be incorpo¬ 
rated in the aquatic plants, epiphytes, and plankton in the 
water column. In climates where senescing of plants oc¬ 
curs in the fall, the majority of the phosphorus taken up will 


be released back into the water column (Figure 3-8). In 
this figure, during the second year Marsh 1 received tap 
water without phosphorus, whereas Marsh 3 received 
wastewater at a loading rate of 0.15 kg P/ha/d throughout. 
Release in excess of influent phosphorus is noted in the 
Marsh 3 effluent in the fall. The effluent phosphorus exhib¬ 
ited for Marsh 1 is the result of release from the standing 
crop developed the year before. Note a background re¬ 
sidual phosphorus concentration of about 0.5 mg/L SRP 
for this system. Maximum removal of phosphate was found 
to be about 1.5 mg/L at a loading of less than 1.5 kg P04/ 
ha/d and reduced to negligible (<0.2 mg/L) at loadings 
above 5 (Gearheart, 1993). This maximum is consistent 
with the theory of Stumm (1975), which suggested approxi¬ 
mately 20% of influent P could be removed under an equi¬ 
librium condition in a lake. 

3.5.3 Phosphorus in Vegetative 
Submerged Beds 

The removal of phosphate from VSB systems relies on 
accretion of phosphorus from decomposing plants and from 
separated, recalcitrant phosphate from the influent to the 
process. Phosphate loading to these systems is large rela¬ 
tive to plant uptake, and reliable sustained removal by har¬ 
vesting of plants prior to senescence would not provide 
significant removal. Accretion of partially decomposed plant 
tissue may provide some additional removal of phospho¬ 
rus. Cycling of phosphorus will produce seasonal effluent 
variations similar to those seen for FWS systems. Some 
minerals associated with the media can provide tempo- 


47 



















































rary removal by means of precipitation/exchange/sorption 
mechanisms, but these effects would be short term (<1 
year) and dependent on the source of the granular materi¬ 
als. 

3.6 Mechanisms of Pathogen Separations 
and Transformations 

3.6.1 Description and Measurement 

Waterborne pathogens including helminthes, protozoans, 
fungi, bacteria, and viruses are of great concern in assess¬ 
ing water quality. Since routine examination for pathogenic 
organisms is not recommended because of cost and the 
low numbers of a specific pathogen present at any given 
time, indicator organisms are used. The most common in¬ 
dicators of the level of waterborne pathogen contamina¬ 
tion in water are the coliform group. Today, the fecal coliform 
test is considered to be a better indicator of human fecal 
contamination than the more general total coliform proce¬ 
dure. Even so, the fecal coliform test is not specific and 
can produce false positive results for human contamina¬ 
tion, since these organisms are excreted by a number of 
warm-blooded animals including those residing in wetland 
environments. Fecal streptococci analysis may also be 
used as an additional indicator of fecal pollution. Together 
with the fecal coliform test, these two procedures are some¬ 
times used to discriminate between human and other warm¬ 
blooded animals. Table 3-1 indicates typical ranges of the 
indicator organisms in typical influents to treatment wet¬ 
land systems. 

Separation of pathogens (and indicators) from the water 
column does not in itself mean that the organisms are no 
longer viable. They may be released from the matrix to 


which they are attached and become available again in 
the water column as infectious agents. The true removal 
of pathogens is only achieved by rendering them nonvi- 
able. 

3.6.2 Fate in Constructed Wetlands 

Pathogens (and indicators) entering wetlands may be 
incorporated within the TSS or may be found as suspen¬ 
sions in the influent wastewater. Those associated with 
TSS would be separated from the water column by the 
same mechanisms as discussed for TSS (sedimentation, 
interception, and sorption). Once separated, the viable 
organisms may be released from the solid matrix and be 
retained within the biofilm or sediment pore-water, or they 
may be reentrained into the water column. Regardless of 
their location, they must compete with the consortium of 
organisms surrounding them. As intestinal organisms, they 
will normally require a rich substrate and high temperature 
to favorably compete. Most will not survive in this compe¬ 
tition. They will also be destroyed by predation or, if near 
the open water surface, by UV irradiation. 

Removal of pathogens (indicators) in wetlands appears 
to be correlated with TSS removal and hydraulic residence 
times (Gearheart et al., 1999; Gersberg et al., 1989). Few 
studies have been performed on the effect of wetlands on 
specific pathogens, but Gearheart has found similar re¬ 
movals to FC with Salmonella and MS2 coliphage. Many 
pathogens are more sensitive to the wetland environment 
than indicators, but some viruses and protozoans (spores) 
may be more resistant. Erratic results have been reported 
on viruses, and mechanisms affecting their removal may 
be different than those that destroy indicators. 


CO 

g 

V > 

3 


Q. 

</> 

o 



Date 


Figure 3-8. Phosphorus pulsing in pilot cells in Areata; Marsh 1 received tap water until June 1982 (no phosphorus load), while Marsh 3 received 
oxidation pond effluent (Gearheart, 1993) 


48 













It is significant to note that indicator organisms and per¬ 
haps pathogens may be generated within the wetland. Thus 
background levels of indicators will be found even in natu¬ 
ral wetland systems (see Table 3-5). These background 
levels are variable, influenced by season and other opera¬ 
tional parameters of the system (Figure 3-9). It should be 
noted that in general these indicator organisms are not 
from human sources. However, constructed wetlands are 
unlikely to consistently meet stringent effluent fecal coliform 
permit levels. Therefore, regulators may require disinfec¬ 
tion of treatment wetland effluents prior to discharge. 
Gearheart (1998) consistently attained an FC count of less 
than 2/100 mL with UV disinfection of FWS effluent. 

3.7 Mechanisms of Other Contaminant 
Separations and Transformations 

3.7.1 Metals 

While some metals are required for plant and animal 
growth in trace quantities (barium, beryllium, boron, chro¬ 
mium, cobalt, copper, iodine, iron, magnesium, manga¬ 
nese, molybdenum, nickel, selenium, sulfur, and zinc), 
these same metals may be toxic at higher concentrations. 
Other metals have no known biological role and may be 
toxic at even very low concentrations (e.g., arsenic, cad¬ 
mium, lead, mercury, and silver) (Gersberg et al., 1984; 
Crites et al., 1997). Influent wastewater to wetlands may 
carry metals as soluble or insoluble species. 

Metals entering wetlands as insoluble suspended solids 
are separated from the water column in a manner similar 
to TSS. Depending on pH and redox potential, these in¬ 
soluble species may be resolubilized and returned to the 
liquid phase. Important removal mechanisms for metals 
include cation exchange and chelation with wetland soils 
and sediments, binding with humic materials, precipitation 


as insoluble salts of sulfides, carbonates, and 
oxyhydroxides, and uptake by plants, algae, and bacteria. 
The chemically bound metals may eventually become bur¬ 
ied in the anoxic sediments where sulfides occur. These 
bound metals are often not bio-available and remain re¬ 
moved from the system. If sediments are disturbed or re¬ 
suspended and moved to oxic regions of the wetland, se¬ 
questered metals may resolubilize. 

Metals may be incorporated into the wetland biomass 
by way of the primary production process. For macro¬ 
phytes, metals are taken up through the root system and 
distributed through the plant. The extent of uptake is de¬ 
pendent on the metal species and plant type. Gersberg et 
al. (1984), found only minor uptake by plants in VSB sys¬ 
tems, while others claim that metals can be found on root 
surfaces due to precipitation and adsorption. The accu¬ 
mulation of heavy metals was found to be variable in a 
marsh in New Jersey receiving wastewater (Simpson et 
al., 1981; 1983). Cadmium, copper, lead, nickel, and zinc 
had accumulated in the litter at the end of the growing sea¬ 
son in much higher concentrations than in the live vegeta¬ 
tion. Other studies have shown that metals like cadmium, 
chromium, copper, lead, mercury, nickel, and zinc can be 
sequestered by wetland soils and biota or both (Mitsch 
and Gosselink, 1993). The high uptake of selenium by biota 
in a wetland marsh receiving irrigation waters was dis¬ 
cussed in Hammer (1992), but some could have been vola¬ 
tilized. Studies have shown that some algae will seques¬ 
ter selected metals (Kadlec and Knight, 1996). Floating 
plants such as duckweed have been shown to be excel¬ 
lent accumulators of cadmium, copper, and selenium, but 
only moderate accumulators for chromium and poor accu¬ 
mulators for nickel and lead (Zayed et al., 1998). A review 
of metal removal in wetlands is found in Kadlec and Knight 
(1996). 


^ 10,000 


o 

o 


<D 

n 

E 

3 

C, 

o 


c 

<D 

3 

3= 

LU 


1,000 - 



100 


100 1,000 

Influent FC (number/100 mL) 


i-r 

10,000 100,000 1 , 000,000 


Figure 3-9. Influent versus effluent FC for the TADB systems (EPA, 1999) 


49 









At the present time there is insufficient long-term data 
on full-scale constructed wetlands to provide a reliable 
estimate of performance on the removal of metals from 
wastewater. However, in VSBs and in fully vegetated FWS 
systems, the anaerobic conditions are conducive to retain¬ 
ing most metals with the settled TSS and minimizing 
resolubilization. Similarly, the actual removals will be af¬ 
fected by the speciation of the influent metals. 

3.7.2 Other Organic Compounds 

There is concern about the fate of many trace organic 
compounds in the environment. These include pesticides, 
fertilizers, process chemicals, and others that fall under 
the category of priority pollutants. The fate of these com¬ 
pounds in wetlands is dependent on the properties of the 
compound, the characteristics of the wetland, the species 
of plants, and other environmental factors. The most im¬ 
portant separation and transformation mechanisms in¬ 
volved include volatilization, sedimentation/interception, 
biodegradation, adsorption, and uptake. These mecha¬ 
nisms have been discussed previously. Recalcitrant organ¬ 
ics that have been separated may accumulate in the wet¬ 
land sediments. Some may be taken up by plants and be 
returned to the system upon plant decomposition. Biodeg¬ 
radation of some organic compounds may result in com¬ 
pletely mineralized end products, or the process may pro¬ 
duce end products that may be more toxic than the parent 
compound. At this time, there is insufficient data available 
on full-scale wetland systems to evaluate how effective 
they are in the long-term removal and destruction of most 
priority pollutants. Based on pretreatment performance, 
oxidation or facultative lagoons remove a high %age of 
volatile and semivolatile organic compounds (Hannah et 
al., 1986), resulting in low influent concentrations to the 
FWS system that follows, while primary sedimentation is 
less effective and results in higher influent concentrations 
of both to subsequent VSB systems. 

3.8 Constructed Wetland Modeling 
3.8.1 Modeling Concepts 

The modeling of wastewater treatment operations and 
processes has long been of interest to environmental en¬ 
gineers. This interest stems primarily from a need to quan¬ 
tify the performance of the process and a desire to opti¬ 
mize the design and operation of the treatment facility. 
Modeling of many of the treatment processes used today 
has met with only partial success primarily because of the 
lack of rigor in most models. This is due to the enormous 
complexity of the reaction mechanisms that may take place 
within many of these systems and with the difficulty in char¬ 
acterizing the constituents within the wastewater. Con¬ 
structed wetlands fall in this category of a highly complex 
system in which a multiplicity of reactions and reaction 
mechanisms occur, even in the simplest of systems. Ad¬ 
sorption, sedimentation, flocculation, biological catalysis, 
precipitation, exchange, and diffusional processes are but 
a few of the important functional mechanisms that may 
control the removal of a given constituent. Furthermore, 
these mechanisms are dependent on a number of physi¬ 


cal, chemical, and biological variables within the system 
(e.g., temperature, redox potential, pH, plant density, etc.). 

It is apparent that in a highly complex system such as 
the constructed wetland, the quantification of all specific 
rate-controlling mechanisms seems unlikely. The transient 
nature of the influent wastewater characteristics and the 
lack of substantial control over the process undoubtedly 
will result in frequent changes of the rate-controlling mecha¬ 
nisms of the process. 

In developing a model for any process, the first question 
that should be asked is, “What is the value of modeling 
this particular system?” Currently, the design engineer may 
be using empirical models that assist in interpolating infor¬ 
mation from lab-scale or pot-scale studies. The major prob¬ 
lem is scale-up, and most of the designer’s time is spent 
on concerns about operating conditions within rather nar¬ 
row ranges in order to meet permit requirements. Added 
to that is the ever-present shortcoming of analytical and 
sampling methods. The answer to the question would ap¬ 
pear to be that there is little value in developing more em¬ 
pirical curve-fitting models for the process. The design 
engineer might be well advised at the present time to de¬ 
velop performance parameters, curves, and operational 
charts that could be used for the plants under investiga¬ 
tion. If a deterministic model of mechanistic proportions is 
to be developed, it needs to be developed through a se¬ 
ries of rigorous processes ensuring that it is, in fact, a true 
model of the process. 

Writing a mechanistic, mathematical model is generally 
easier than verifying it. That is not to say that conceiving 
and writing the model is trivial, but rather to emphasize the 
difficulty in giving the model a fair chance to fail the experi¬ 
mental test. Note that the emphasis is on a chance for 
verification failure rather than a chance to pass. Thus an 
investigator’s problem of ownership of a particular model 
presents itself, which is not easily subjugated to the scien¬ 
tific issue of fairly and rigorously testing the model. 

The experimental learning process is basically iterative 
and consists of successive and repeated use of the se¬ 
quence. Box and Hunter (1965), although not the first to 
note this important underlying pattern in experimentation, 
have best exploited the pattern and developed strategies 
for experimentation that efficiently lead one through the 
cycle and advise one in determining the details of subse¬ 
quent cycles. One such cycle is illustrated in Figure 3-10. 
In environmental engineering, a field that relies heavily on 
empirical methods, the unavoidable long-term character 
of the experiments required for a system, such as a con¬ 
structed wetland, has resulted in many different individu¬ 
als conducting rather inefficient iterations of different parts 
of the problem. As a result, the development of a rigorous 
model of the process, or parts of it, has not been achieved 
as is true with many of the wastewater treatment processes 
designed today. 

Of particular importance in the iterative sequence are 
the steps of design and analysis. In this context, design is 


50 





Model ◄- 

i 

Reliable and Pertinent ◄- 

Data 

4 

Parametric Estimates 

i 

Quality of Fit 

i 

Adequacy of Model 

I I 

Yes No 

i 

Experimentation - 

Figure 3-10. Adaptive model building 


devising experiments suggested by the current status of 
the process that will most likely improve an understanding 
of it. Analysis would be the examination of the data in such 
a manner as to discover how far conjectures are born out 
and spark modified or new conjectures when the old ones 
are found wanting. Both engineering and statistics play an 
important role in the process. 

3.8.2 Status of Wetland Modeling 

What is the status of model building for constructed wet¬ 
lands at this time? There are a number of ways one might 
classify models. One classification would include linguis¬ 
tic models, stochastic models, and deterministic models. 
Linguistic, or word, models are qualitative and have as their 
main advantage the capability of representing incomplete 
states of knowledge not explicitly represented by the other 
two types of models. Stochastic models of processes are 
the result of data-driven, top-down approaches to model¬ 
ing and can reflect information contained in the data used 
to prepare them. These models are useful but are data 
intensive and require that the data be representative of 
the behavior of the process that they are designed to model. 
Deterministic models, both empirical and mechanistic, are 
either built from the bottom up based on first principles or 
result from careful observation of phenomena. They are 
robust and general, and are effective provided that the data 
are available to support the theory and to calibrate the 
model. 


An examination of the literature on constructed wetlands 
suggests that all three of these models have been pro¬ 
posed or used. Yet it is worth noting that none have really 
met the test of the iterative process. There are very few 
full-scale studies of constructed wetlands that have pro¬ 
vided a database sufficient to demonstrate success or fail¬ 
ure of a given model. Most studies lack sufficient spatial 
and temporal sampling to even identify whether a model 
fails the experimental test of verification. In an effort to pro¬ 
vide more data, investigators use databases from a num¬ 
ber of sites. Site-to-site variability makes this process sus¬ 
pect. Quality control on the data collected is also of con¬ 
cern. For the most part, the data fit to constructed wet¬ 
lands is regressed from empirical models, and fit is ex¬ 
pressed in terms of the coefficient of determination, R 2 . It 
is important to note that a high value of R 2 does not assure 
a valid relation nor does a low value mean that the model 
is useless. It is often stated that R 2 explains a certain pro¬ 
portion of the variability in the observed response. If the 
data were from a well-designed controlled experiment, with 
proper replication and randomization, it is reasonable to 
infer that a significant association of the variation in y with 
variation in the level of x is a causal effect of x. If, however, 
as in the case of most wetland data, the data are observa¬ 
tional, there is a high risk of a causal relationship being 
wrong. With observational data, there can be many rea¬ 
sons for associations among variables, only one of which 
exhibits causality. Totally spurious correlations, often with 


51 







high R 2 values, can arise when unrelated variables are 
combined. 

Often rival models may be tentatively considered for a 
constructed wetland process. It is not uncommon to find 
that more than one model can be calibrated to fit the data 
and give residual errors that are acceptable. In some cases, 
the selection of the seemingly acceptable model is of little 
practical consequence over the range of interest. In other 
cases, knowing which model is better may throw light on 
fundamental questions about reaction mechanisms and 
other phenomena under investigation. A fundamental con¬ 
cept in model discrimination is that rival models often di¬ 
verge noticeably only at extreme conditions. Thus extremes 
must be evaluated to provide useful statistical information. 
Models must be put into jeopardy of failing. An excellent 
discussion of this may be found in Berthouex and Brown 
(1994). 

In summary, it appears premature, given the lack of qual¬ 
ity-assured data, for designers of constructed wetlands to 
rely on regressed empirical models. The problem is one of 
extrapolation of these mostly “black box” models from site 
to site and extension of the model outside of the database. 
Examination of the wide variation in parameter values (re¬ 
action coefficients, background concentrations, etc.) sug¬ 
gests that fitting data to simplistic models may be insuffi¬ 
cient at this time to provide reliable design information in 
most cases. Currently the design of these systems should 
be based on design parameters (e.g., hydraulic loading, 
nitrogen loading, detention time, etc.) and operating crite¬ 
ria that are required to meet a specific effluent limitation. 
This is not a novel approach, but has been used for many 
decades by environmental engineers in the design of highly 
complex unit processes including the activated sludge pro¬ 
cess and waste stabilization ponds. Only when a carefully 
designed series of iterative studies have been conducted, 
and data based on quality-controlled specifications have 
been analyzed, can rigorous models be provided for use 
in wetland system design. 

Currently there is a North American Database (NADB) 
on wetlands that has been relied on for purposes of wet¬ 
land design. Efforts have been made to refine that data¬ 
base to improve reliability. What is urgently needed at this 
time is an effective plan for the design of studies that will 
provide a comprehensive understanding of the processes 
that occur within wastewater wetlands. Efforts have already 
been made to mathematically model some of the wetland 
processes based on first principles. These models should 
serve as the starting point for the adaptive iterative pro¬ 
cess as described in Figure 3-10. The experimental de¬ 
sign should include extensive, quality-assured, transect 
data at numerous selected sites (spatial variations) over 
an extended period of time (temporal variations). Charac¬ 
terization of the wastewater must include both particulate 
and soluble fractions of contaminants and must ensure 
quality control of both sampling and analyses. Character¬ 
ization of the wetland is also important and should include 
data on residence time distribution of flow, geometry, plant 
species and distribution, monolithic zone coverage and 


distribution, and so forth. Once this database is developed, 
the iterative modeling of this very complex system can 
begin in earnest. Given the number of years of constructed 
wetland experience, such efforts are generally overdue. 

3.9 References 

Armstrong, J. and W. Armstrong. 1990. Light enhanced 
connective throughflow increases oxygenation in rhi¬ 
zomes & rhizospheres of Phragmites australis (Cav.). 
Trin. Ex. Steud., New Phytologist, 114:121. 

Barber, L., et al. 1999. Transformation of dissolved organic 
carbon in constructed wetland systems. Project Re¬ 
port, U.S. Dept. Interior, Bureau of Reclamation. 

Bayley, R.W., E.V. Thomas, and P.F. Cooper. 1973. Some 
problems associated with the treatment of sewerage 
by non-biological processes. In: Applications of new 
concepts in physical-chemical wastewater treatment. 
W.W. Eckenfelder and L.K. Cecil (eds.). Oxford, UK: 
Pergamon Press, pp. 119-132. 

Bella, D. 1970. Simulating the effect of sinking and vertical 
mixing of algae population dynamics. J. Water Poll. 
Cont. Fed. 42(2):R140. 

Berthouex, P.M. and L. Brown. 1994. Statistics for envi¬ 
ronmental engineers. Boca Raton, FL: Lewis Publica¬ 
tions Inc., 287 pp. 

Box G.E.P. and W.G. Hunter. 1965. The experimental study 
of physical mechanisms. Technometrics, 7:23. 

Brix, H. and H. Schierup. 1990. Soil oxygenation in con¬ 
structed reed beds: The role of macrophyte and soil- 
atmosphere interface oxygen transport. In: P.F. Coo¬ 
per and B.C. Findlater (eds.) Proceedings of interna¬ 
tional conference on use of constructed wetlands in 
water pollution control. Oxford, UK: Pergamon Press. 

Burgoon, P.S., K.R. Reddy, T.A. DeBusk, and B. Koopman. 
1991. Vegetated submerged beds with artificial sub¬ 
strates. II: N and P Removal. Journal ASCE-EED, 
117(4):408-424. 

Camp, T.R. and P.C. Stein. 1943. Velocity gradients and 
internal work in fluid motion. J. Boston Soc. Civ. Eng., 
30:209. 

Crites, R.W., et al. 1997. Removal of metals and ammonia 
in constructed wetlands. Water Environment Research, 
69(2). 

Gearheart, R.A. 1993. Phosphorus removal in constructed 
wetlands. Paper No. AC93-023-001. Presented at the 
66th Annual WEFTEC Meeting, Anaheim, CA. 

Gearheart, R.A. 1998. The use of free surface constructed 
wetland as an alternative process treatment train to 
meet unrestricted water reclamation standards. In: 
AWT98 Proceedings, p. 559. 


52 





Gearheart, R.A. and B.A. Finney. 1996. Criteria for design 
of free surface constructed wetlands based upon a 
coupled ecological and water quality model. In: Pro¬ 
ceedings of 5th International Conference on Wetland 
Systems for Water Pollution Control. IAWQ, Vienna, 
Austria. 

Gearheart, R.A., et al. 1983. City of Areata marsh pilot 
project, effluent quality results—System design and 
management. Project No. C-06-2270. State Water 
Resources Control Board, Sacramento, CA. 

Gearheart, R.A., F. Klopp, and G. Allen. 1989. Constructed 
free water surface wetlands to treat and receive waste- 
water: Pilot project to full scale. In: Constructed wet¬ 
lands for wastewater treatment, D.A. Flammer (ed.) 
Chelsea, Ml: Lewis Publishers, Inc., pp. 121-137. 

Gearheart, R.A., et al. 1999 [In draft]. Free water surface 
wetlands for wastewater treatment: A technology as¬ 
sessment. U.S. Environmental Protection Agency, Of¬ 
fice of Water Management, U.S. Bureau of Reclama¬ 
tion, Phoenix, AZ. 

Gersberg, R.M., et al. 1984. The removal of heavy metals 
by artificial wetlands. In: Proceedings Water Reuse 
Symposium, III. Vol. 2. AWWA Research Foundation, 
p. 639. 

Gersberg, R.M., et al. 1986. Role of aquatic plants in waste- 
water treatment by artificial wetlands. Water Research 
20(3):363. 

Gersberg, R.M., R.A. Gearheart, and M. Ives. 1989. Patho¬ 
gen removal in constructed wetlands. In: Proceedings 
from First International Conference on Wetlands for 
Wastewater Treatment. Chattanooga, TN, June 1988. 
Ann Arbor, Ml: Ann Arbor Press. 

Godschalk, G.L. and R.G. Wetzel. 1978. Decomposition 
in littoral zones of lakes. In: R.E. Good et al. (eds.) 
Freshwater wetlands: ecological process and manage¬ 
ment potential. New York, NY: Academic Press. 

Grady, Jr., C.P.L. and H.C. Lim. 1980. Biological wastewa¬ 
ter treatment—Theory & applications. New York, NY: 
Marcel Dekker, Inc. 

Hammer, D.A. 1992. Creating freshwater wetlands. 
Chelsea, Ml: Lewis Publishers, Inc., 298 pp. 

Hannah, S.A., B.M. Austern, A.E. Eralp, and R.H. Wise. 
1986. Comparative removal of toxic pollutants by 6 
wastewater treatment processes. Jour. WPCF, 
58(1):27-34. 

Heukelekian, H. and J.L. Balmat. 1959. Chemical compo¬ 
sition of the particulate fractions of domestic sewage. 
Sewage Ind. Wastes, 31 (4):413. 

Hutchinson, G. 1967. A treatise on limnology. Vol. 2. New 
York, NY: John Wiley & Sons. 


Kadlec, R.H. and R.L. Knight. 1996. Treatment wetlands. 
Boca Raton, FL: Lewis-CRC Press. 

Levine, A.D., G. Tchobanoglous, and T. Asano. 1984. Char¬ 
acterizations of the size distribution of contaminants 
in wastewater: Treatment and reuse implication. 57th 
Annual Conference Water Pollution Control Federa¬ 
tion, New Orleans, LA. 

Levine, A.D., G. Tchobanoglous, and T. Asano. 1991. Size 
distribution of particulate contaminants in wastewater 
and their impact on treatability. Wat. Res., 25(8):911 — 
922. 

Lewin, R.A. (ed.) 1962. Physiology & biochemistry of al¬ 
gae. New York, NY: Academic Press, 751 pp. 

Liehr, S.K. 2000. Constructed wetlands treatment of high 
nitrogen landfill leachate. WERF Report No. 94-IRM- 
U. NCSU. 

Lienard, A. 1987. Domestic wastewater treatment in tanks 
with emergent hydrohoytes: Latest results of a recent 
plant in France. Water Sci. Tech., 19(12):373-375. 

Metcalf and Eddy. 1991. Wastewater engineering. 3d ed. 
New York, NY: McGraw-Hill, Inc. 

Mitsch, W.J. and J.G. Gosselink. 1993. Wetlands. New 
York, NY: Van Nostrand Reinhold. 

Munch R., C.P. Hwang, and T.H. Lackie. 1980. Wastewa¬ 
ter fractions add to total treatment picture. Water Sew. 
Wks., 127:49-54. 

O’Connor, D.J. and W.E. Dobbins. 1958. Mechanism of 
reaeration in natural streams. Trans. Amer. Soc. Civil 
Eng., 123:641. 

Reddy, K.R. and D.A. Graetz. 1988. Carbon and nitrogen 
dynamics in wetland soils. In: D.D. Hook et al. (eds.) 
The ecology and management of wetlands. Vol. I. Port¬ 
land OR: Timber Press, p. 307. 

Reed, S.C., R.W. Crites, and E.J. Middlebrook. 1995. Natu¬ 
ral systems for waste management and treatment. 2d 
ed. New York, NY: McGraw-Hill. 

Rickert, D.A. and J.V. Hunter. 1972. Colloidal matter in 
wastewater secondary effluents. J. Water Pollution 
Control Federation, 44(1):134. 

Rose, C. and W.G. Crumpton. 1996. Effects of emergent 
macrophytes and dissolved oxygen dynamics in a prai¬ 
rie pothole wetland. Wetlands, 16(4):495. 

Simpson R. L., et al. 1981. Dynamics of nitrogen, phos¬ 
phorus, and heavy metals in Delaware River freshwa¬ 
ter tidal wetland. U.S. Environmental Protection 
Agency, Final Report, Corvallis Environmental Re¬ 
search Lab, Corvallis, OR. 


I 


53 









Table 4-1. Loading and Performance Data for Systems Analyzed in this Document (DMDB). 


Constituent 

Min 

Pollutant Loading 
Rate (kg/ha-day) 
Mean 

Max 

Min 

Influent (mg/L) 
Mean 

Max 

Min 

Effluent (mg/L) 
Mean 

Max 

BOD, 

2.3 

51 

183 

6.2 

113 

438 

5.8 

22 

70 

TSS 

5 

41 

180 

12.7 

112 

587 

5.3 

20 

39 

nh 4 -n 

0.3 

5.8 

16 

3.2 

13.4 

30 

0.7 

12 

23 

TKN 

1.0 

9.5 

20 

8.7 

28.3 

51 

3.9 

19 

32 

TP 

— 

— 

— 

0.56 

1.39 

2.41 

0.68 

2.42 

3.60 

FC 

— 

— 

— 

42000 

73000 

250000 

112 

403 

713 


BOD = Biochemical Oxygen Demand (5 day) 

TSS = Total Suspended Solids 

NH 4 -N = Ammonia Nitrogen 

TKN = Total Kjeldahl Nitrogen 

TP = Total Phosphorus 

FC = Fecal Coliform, cfu/IOOmL 


ance of constituents, e.g., not all systems reported all con¬ 
stituents. 

4.1.3 BOD Performance 

The relationship between average biochemical oxygen 
demand (BOD) loading and average BOD effluent con¬ 
centration for the studied systems is shown in Figure 4-1. 
. There is a general trend between increased BOD loading 
and increased effluent concentration up to the highest load¬ 
ing of 183 kg/ha-day if only the fully vegetated designs are 
taken into account. The figure reveals considerable efflu¬ 
ent variation for a given BOD loading and shows consider¬ 
able variation in effluent quality at the lower BOD loading 
rates. The effect of the background BOD, due to release 
from previously settled influent TSS and plant decomposi¬ 
tion is especially evident in systems with low loading rates. 
Figure 4-2 illustrates the internal BOD load which occurs 
from the partial anaerobic digestion of previously settled 
organic solids and plant exudates and other byproducts of 
anaerobic biodegradation. The portion of that internal load¬ 
ing which is due to the plant detritus has been measured 
and is illustrated in Figure 4-3. Since anaerobic processes 
are extremely sensitive to temperature, these internal load¬ 
ings begin in the spring as water temperature rises and 
continue until the backlog of settleable organics and plant 
detritus accumulated over the winter is exhausted. Plant 
exudates and senescent byproducts occur in their own 
cycles which provide internal loading to the system in a 
dynamic manner. Background BOD concentrations can 
range up to almost 10 mg/L. 

A more conservative analysis of Figure 4-1 indicates that 
a fully vegetated FWS should not be loaded above 40 kg 
BOD/ha-d if a secondary effluent BOD standard (30 mg/L) 
is to be met. Restricting the analysis to open water FWS 
systems indicates that loadings up to 45 kg/ha-d yielded 
effluent BOD of less than 20 mg/L, and loadings up to 130 
kg/ha-d always met this quality. However, there is only one 
data point above 45 kg/ha-d and it is at 130 kg/ha-d. The 
open water FWS systems which permit reaeration and 
aerobic oxidation should be designed for an areal loading 
of no more than 60 kg/ha-d to consistently attain an efflu¬ 


ent BOD of <30 mg/L until more performance data are ob¬ 
tained. Coincidently, Stowell (1988) recommended an up¬ 
per limit of 60 to 70 kg BOD/ha-d to prevent odors from an 
FWS system. Open water FWS systems loaded below 45 
kg/ha-d can be expected to attain effluent BODs of 20 mg/ 
L or less. Analysis of the TADB yields a similar maximum 
areal BOD loading rate of 50 kg/ha-d without differentia¬ 
tion between fully vegetated and open water FW systems 
(EPA, 1999). 

4.1.4 TSS Performance 

The effectiveness of FWS treatment wetlands to remove 
total suspended solids (TSS) is recognized as one of their 
principal advantages. Over a range of loadings from 5 to 
180 kg/ha-day, there are several relationships between 
loading and effluent TSS quality with the DM data, as shown 
in Figure 4-4. Under a fairly narrow range of solids load¬ 
ings, (up to 30 kg/ha-d) secondary effluent TSS concen¬ 
trations (< 30 mg/L) can be attained with fully vegetated 
systems. Since physical processes dominate the removal 
of TSS, similar designs should produce similar effluent 
qualities. Analysis of the TADB (EPA, 1999) yields similar 
maximum loading. 

TSS removal is most pronounced in the inlet region of a 
FWS constructed wetland. Generally, the influent TSS from 
oxidation pond systems are removed in the first 2 to 3 days 
of the nominal hydraulic retention time in fully vegetated 
zones near the inlet (Gearheart.et al, 1989; Reed, et al, 
1995; Kadlec and Knight, 1996). Enhanced settling and 
flocculation processes account for most of this removal, 
and the overall removal efficiency is a function of the ter¬ 
minal settling velocity of the influent and flocculated sol¬ 
ids. Long-term removal of detrital material will likely be re¬ 
quired 10-15 years into operation. The separated solids 
undergo anaerobic decomposition, releasing soluble dis¬ 
solved organic compounds and gaseous by-products, car¬ 
bon dioxide and methane gas, to the water column. Figure 
4-2 shows the reduction of total and soluble BOD and TSS 
through a pilot project wetland cell. Approximately 80% of 
the TSS is removed in the first two days of theoretical HRT 
primarily due to enhanced sedimentation and flocculation. 


56 









80 


70 - 


❖ 

V 


60 - 



Q 

O 

CD 


c 

<D 

3 


LU 


50 - 


40 - 


30 - 


20 - 


V ♦ 


V 


❖ 

V 


10 


0 


❖ 


o 

o 

V 


00 


❖ 

o 


♦ 

o 



♦♦ 

vv 



♦ 

V 


❖ 

V 


o 

V 


V = Fully Vegetated 
O = Significant Open Area 


0 


50 


100 


150 


BOD Load (kg/ha*day) 

Figure 4-1. BOD effluent vs. areal loading (DMDB) 



Figure 4-2. Release of soluble BOD during early stage 


200 


57 















Percent Dry Weight of Standing Crop to 
Water Column 

Figure 4-3. Annual detritus BOD load Scripus and Typha - assuming 15,000 kg/Ha standing crop (Gearheart & Finney, 1996) 


50 


40 - 


V 


05 

E. 

CO 

CO 


c 

a> 

3 


30 - 


£ 20 


10 - 


♦ 

V 


V 


♦ 

o 


❖ 

o 


oo 


V 


♦ 

V 


0 

V 


o 

V 


o O 
V 



T 

50 


o 

V 


o 


V 


o 

V = Fully Vegetated 
O = Significant Open Area 


- 1 - 1 — 

100 150 

TSS Load (kg/ha-day) 


Figure 4-4. TSS loading vs. TSS in effluent (DMDB) 


200 


58 































































Subsequently, the removal essentially ceases without sub¬ 
sequent open zones which can provide conditioning and 
transformation processes which may improve overall re¬ 
moval of TSS attainable by the system. 

A closer analysis of the DMDB again shows that TSS 
loadings can be higher for FWS systems with open-water 
zones. Only one very small site with such zones exceeded 
secondary effluent TSS standards (30 mg/L), and it was 
loaded at more than 90 kg/ha-d. Below a loading of 30 kg/ 
ha-d an effluent of 20 mg/L of TSS was consistently achiev¬ 
able. It is therefore recommended that in addition to that 
areal loading limitation a maximum loading of 50 kg/ha-d 
be employed to attain an effluent of 30 mg/L of TSS until 
more performance data can be obtained. 

4.1.5 Nitrogen Performance 

Any discussion of nitrogen species in FWS constructed 
wetlands must be predicated by a return to first principles, 
as described in Chapter 3. Given the numerous transfor¬ 
mation possibilities and the dearth of removal mechanisms, 
there are only a few meaningful explanations. Influent ni¬ 
trogen for the typical applications of this manual will be 
primarily in the form of ammonia-nitrogen (NFI4-N) with a 
significant organic nitrogen (ON) contribution. Approxi¬ 
mately 10 to 15% of the oxidation pond effluent TSS due 


to algae is organic nitrogen (Balmer and Vik, 1978). Since 
both are measured by the total Kjeldahl nitrogen (TKN) 
test, it becomes the likely standard of areal loading analy¬ 
sis. Any discussion of just one species (typically, NH4-N) 
is of little value and often misleading to readers. 

Another key discussion point is the inherent inability of 
fully vegetated FWS systems to nitrify a typical FWS influ¬ 
ent, as described in the preceding paragraph, within a prac¬ 
tical number of days of HRT. During periods of senescence 
when fully vegetated zones become partially open-water 
zones, different mechanisms of treatment can replace 
these which dominate during the normal growing season, 
if the climate can sustain them. This is further reinforce¬ 
ment for the fact that there are few absolutes in natural 
systems. Sadly, these conditions are rarely recognized or 
adequately described and measured, making use of most 
existing data sets open to some question. However, in this 
analysis the fact that a system is classified as either fully 
vegetated or as having significent open-water zones aids 
in explaining many anomolies. 

4.1.5.1 TKN Performance 

Figure 4-5 illustrates that for a fully vegetated FWS which 
receives 30 to 50 mg/L of TKN the effluent exceeds 24 
mg/L since the only removal mechanism is sedimentation 


35 


CT> 

E 

z 

* 


c 

4 ) 

3 

LU 


30 - 


25 - 


20 - 


15 


10 - 


5 - 


♦ 

V 


♦ 

v 


♦ 

v 


« 

v 


♦ 

v 


❖ 

V 


❖ 

V 


♦ ♦♦ 

ooo 


T 

5 


T 

10 


T 

15 


V = Fully Vegetated 
O = Significant Open Area 


20 


25 


TKN Load (kg/ha-day) 


Figure 4-5. Effluent TKN vs. TKN loading 


59 




of organic nitrogen (ON). One more lightly loaded (3.3 kg/ 
ha-d) fully vegetated system did produce an effluent TKN 
of about 9 mg/L. The three open-water zone systems were 
all lightly loaded (< 2.8 kg/ha-d) and produced effluent TKN 
of about 4 mg/L. Since these latter designs offer a mecha¬ 
nism to transform the TKN to nitrate-nitrogen (N03 -N), 
which could subsequently be removed through denitrifica¬ 
tion, they could conceivably be loaded more heavily and 
still meet a stringent total nitrogen standard (e.g., 10 mg/ 
L). No more heavily loaded systems were indicated in the 
DMDB. The TADB analysis (EPA, 1999) did indicate that 
any FWS system which received a TKN loading of less 
than 3.3 kg/ha-d could meet an effluent TKN of less than 
10 mg/L, but does not subdivide results into the two 
subcatagories used herein. Arcata’s open-water-zone sys¬ 
tems have been able to maintain effluent total nitrogen (TN) 
below 5 mg/L at loadings of up to 3 kg TN/ha-d (Gearheart, 
1995) through the denitrification provided near the FWS 
system outlet which is fully vegetated. Maximum TKN load¬ 
ings to sustain an effluent TKN of less than 10 mg/L can 
conservatively be set at 5 kg/ha-d until more data are made 
available. This only applies to open water FWS systems, 
while fully vegetated systems are limited to only a small 
percentage of TKN removal due to the settling of organic 
nitrogen particulate matter. 

4.1.5.2 Denitrification 

The extent of nitrate removal via denitrification is de¬ 
pendent on the extent of the prior conversion of TKN to 
N03-N, a labile carbon or other energy source and anaero¬ 
bic/anoxic conditions in the water column. Therefore, deni¬ 
trification, which converts N03-N to gaseous end prod¬ 
ucts which can leave the constructed wetland, is best suited 
to a fully vegetated condition. Further, any N03-N which 
enters a FWS wetland is likely to be quickly removed, while 
any nitrate formation (nitrification), which occurs in the 
open-water zones of the FWS can be removed near the 
fully vegetated outlet zone if the conditions noted above 
are met. The DMDB offers no assistance in that all the 
systems which had nitrate-nitrogen in their influent had it 
at very low concentrations (average = 2.47 mg/L), even 
though effluent concentrations were lower (average = 2.22 
mg/L). 

Gearheart (1995) reports that the carbon produced from 
decaying macrophytes is sufficient to denitrify 100 mg N03- 
N/L in an FWS and that the reaction rate is temperature 
dependent, signifying its biological nature. Reed, et al 
(1995) and Crites and Tchobanoglous (1998) also indicate 
that FWS systems have the capablility to denitrify, but they 
offer no specific examples. Therefore, denitrification should 
be feasible in FWS systems as long as there is sufficient 
detention time in fully vegetated zones with anaerobic/an¬ 
oxic conditions. 

4.1.5.3 Ammonia Nitrogen Performance 

Also, as noted previously, ammonia-nitrogen (NH4-N) 
limits are often specified for treatment facilities in their 
NPDES permit. However, the level of effluent ammonia in 
an FWS constructed wetland effluent bears only a tenu¬ 


ous relationship to its influent NH4-N concentration. The 
normal case will find that all influent nitrogen is measured 
as TKN, and that this total will be divided between organic 
nitrogen and NH4-N. It is likely that this total nitrogen will 
be reduced in any FWS owing to the loss of organic nitro¬ 
gen due to enhanced settling. In the DMDB the average 
TKN removal was 32%, while the fully vegetated systems 
reported 28%. This difference is not larger because the 
open-water FWS systems were few in number and were 
all loaded more lightly. Although one can plot the effluent 
NH4-N vs NH4-N loading from the DMDB, the data dem¬ 
onstrate no useful relationship. Therefore, unless a FWS 
is designed for very low TKN loadings with an ample open- 
water zone to nitrify the influent, there is no meaningful 
chance to meet any NH4-N effluent standard which is sig¬ 
nificantly lower (>30%) than the influent concentration. 

4.1.5.4 Other Nitrogen Performance 

Since total nitrogen is the sum of all forms of nitrogen, it 
will be reduced through nitrification/denitrification, the loss 
of organic nitrogen due to flocculation and sedimentation 
and plant uptake of NH4-N. Since some of this settled frac¬ 
tion will return to the mainstream as the settled organics 
partially digest and the plant uptake will return due to se¬ 
nescence, the total nitrogen budget should be evaluated 
on an annual basis. The returned nitrogen will likely be in 
the same two forms (organic and ammonia-nitrogen) as 
the normal influent TN load, making this internal load very 
compatible with the external load. Given the transformability 
of individual nitrogen components between each other 
based on the conditions existing at different locations in 
the FWS wetland, the designer needs to provide passive 
controls(e.g..depth and vegetation patterns) if he or she 
wishes to remove a substantial portion of the incoming ni¬ 
trogen load. 

4.1.6 Total Phosphorus Performance 

Only 4 of the DMDB systems measured TP loadings and 
effluent quality, as shown in Figure 4-6. While some ap¬ 
proximate comparisons can be made, the need to sepa¬ 
rate the inorganic phosphorus performance from the or¬ 
ganic particulate phosphorus performance makes the lack 
of DM data impossible to utilize effectively, therefore, the 
TA database (EPA.1999) is used for an approximate analy¬ 
sis. 

Figure 4-7 shows that over a range of loading up to 4.5 
kg/ha-day at the TADB sites, total phosphorus effluent con¬ 
centration generally increased with loading (USEPA, 1999). 
At the lower loading rates (<0.55 kg/ha-d), however, the 
effluent phosphorus concentration was less than 1.5 mg/ 
L. The fractional distribution of TP in municipal wastewa¬ 
ter previously treated by lagoons is variable and has not 
been well documented. Balmer and Vik (1978) found fil¬ 
tered P/total P to be 20-25%, but the flocculation and sedi¬ 
mentation superiority of the initial FWS fully vegetated zone 
will remove a significant % of those forms which enter as 
supracolloidal and settleable solids. Gearheart (1993) has 
performed extensive studies on the Areata FWS systems 
and found a relationship between areal loading and phos- 


60 




Effluent TP (mg/L) 


4 


3.5 - 


❖ 



2.5 - 


♦ 


2 - 


1.5 - 


1 - 


0 0.5 1 1.5 2 2.5 3 

TP Load (kg/ha*day) 


Figure 4-6. Effluent TP vs. TP areal loading 



Figure 4-7. Average total phosphorus loading rate vs. total phosphorus effluent concentration for TADB wetland systems 


61 









phorus removal. An upper limit of 1.5 mg/L removal of or¬ 
thophosphates was found at loadings of less than 1.5 kg/ 
ha-d and a hydraulic retention time (HRT) of at least 15 
days. HRTs of less than 7 days yielded a maximum re¬ 
moval of 0.7 mg/L of orthophosphate, as shown in Figure 
4-8. 

The phosphorus cycle of uptake during the growing sea¬ 
son and release during senescence and the initial sub¬ 
stantial uptake of phosphorous by the soil of the FWS fur¬ 
ther confounds short-time studies of phosphorus transfor¬ 
mations and removal. As a final issue in this conundrum, 
the particulate fractionation and chemical speciation of the 
influent phosphorus will also have some impact on trans¬ 
formations and fate of P. Therefore, long-term studies which 
differentiate between P forms and document vegetation 
condition, climate, temperature and other pertinent water 
quality parameters are necessary to provide meaningful 
information for future FWS system designers, but the an¬ 
nual P removal by these systems is quite limited based on 
mechanistic evaluations and controlled studies. 

4.1.7 Fecal Coliform Performance 

Only four systems in the DMDB reported fecal coliform 
(FC) data, and three of those were fully vegetated sys¬ 
tems. The average removal was just over 2 logs, from 
72,700/1 OOmL to 400/100ml. The TADB (EPA, 1999) re¬ 
sults are shown in Figure 4-9 which offers no obvious rela¬ 
tionship between inlet and outlet concentrations. Figure 4- 
10 (Gearheart, 1989) shows results from the Areata pilot 
study which demonstrate that flocculation/sedimentation 
is the primary FC removal mechanism for fully vegetated 
cells or zones. This figure is especially valuable in light of 
the world literature on FC dieoff in lagoons which identify 
solar radiation as the primary disinfection mechanism 
(Mara, 1975). Such a mechanism can be effective in open- 
water zones of the FWS, but the same mechanisms which 
remove settleable and colloidal solids in fully vegetated 
zones are responsible for FC removal in those zones. 



Figure 4-8. Retention Time vs. Orthophosphate Removal (mg/I) 


Estimates of the internal addition of background fecal 
coliform by wildlife in treatment wetlands are provided by 
those systems that receive disinfected influent. For ex¬ 
ample, the Areata Enhancement Wetland received chlori¬ 
nated effluent, and during the period 1990-1997 
(Gearheart,et al,1998), the effluent FC was less than 500 
MPN/1 OOmL about 80% of the time. This is a system with 
large open-water zones that supports a wide variety and 
high population of aquatic birds and mammals. Higher lev¬ 
els of background FC levels are found during the fall and 
winter bird migration period. A similar study on the same 
system during 1995-1996 showed that the effluent mean 
was 40 CFU/IOOmL, was less than 300 CFU/IOOmL over 
90 percent of the time, and on no occasion exceeded 500 
CFU/1 OOmL. Studies of MS-2 coliphage showed similar (2 
logs) removal to that which was obtained with FC 
(Gearheart, 1995). 

The considerable temporal variability in the effluent mi¬ 
croorganism counts produced by treatment wetlands and 
conventional treatment technologies suggests the use of 
geometric averaging to determine monthly mean values 
from daily or weekly measurements. Even with geometric 
means, individual monthly values are frequently 10 times 
larger or smaller than the long-term mean for many treat¬ 
ment wetlands, possibly due to wildlife habitat features. 
This implies that at sites which have strict FC restrictions, 
the ability to disinfect the FWS effluent is required. 

4.1.8 Metals & Other Particulate-Oriented 
Pollutants 

While some metals are required for plant and animal 
growth in trace quantities (barium, boron, chromium, co¬ 
balt, copper, iodine, iron, magnesium, manganese, mo¬ 
lybdenum, nickel, selenium, sulfur, and zinc), these same 
metals may be toxic at higher concentrations. Other met¬ 
als may be toxic at even very low concentrations (e.g. ar¬ 
senic, cadmium, lead, mercury, and silver) (Gearheart, 
1993). 

Information from FWS treatment wetlands indicates that 
a fraction of the incoming metal load will be trapped and 
effectively removed through sequestration with settleable 
suspended solids and soils. For many metals, the limited 
data indicate that concentration reduction efficiency and 
mass reduction efficiency correlate with TSS reduction. 
Wetland background metal concentrations and internal 
profiles are not well known. It has been shown that chro¬ 
mium levels higher than 0.1 mg/L and copper levels higher 
than 1.0 mg/L have detrimental effects on a floating duck¬ 
weed species (Lemna gibba). Table 4-2 shows metal con¬ 
centration data obtained from a constructed wetland dem¬ 
onstration project in Sacramento, where disinfected acti¬ 
vated sludge effluent was applied to parallel 1.44 acre cells, 
each with a hydraulic loading of 65 m3/ha-d and similar 
plant density. 

The valence and form of each metal was not determined, 
but nickel and arsenic appeared to be the most resistent 
to removal (SCRSD, 1998). Several researchers have stud- 


62 




















~ 10,000 1 


o 

o 


0) 

n 

E 

3 

C, 

o 


c 

0 

_3 

5= 

LU 


1,000 



100 -= 


1 


10 


100 1,000 10,000 100,000 1 , 000,000 

Influent FC (number/100 mL) 

Figure 4-9. Average influent FC concentration vs. FC effluent concentration for TADB wetland systems 



10 5 


10 4 


1,000 


100 



Figure 4-10. Areata pilot Cell 8, TSS, BOD, FC 


Table 4-2. Trace Metal Concentrations and Removal Rates, Sacramento Regional Wastewater 
Treatment Plant (SCRSD, 1998). 


Removal (%) 


Metal 


Influent (mg/L) 



Effluent (mg/L) 


Rate 


Min 

Mean 

Max 

Min 

Mean 

Max 


Silver 

0.25 

0.29 

0.32 

0.02 

0.03 

0.03 

90 

Arsenic 

2.00 

2.23 

2.60 

1.50 

2.20 

3.10 

1.3 

Cadmium 

0.040 

0.077 

0.140 

0.005 

0.009 

0.019 

88 

Chromium 

0.50 

1.05 

1.40 

0.50 

0.77 

3.10 

27 

Copper 

4.60 

8.62 

17.00 

1.60 

4.04 

7.00 

19 

Mercury 

0.0084 

0.0105 

0.0144 

0.0021 

0.0031 

0.0041 

71 

Nickel 

4.30 

8.23 

23.00 

4.10 

8.96 

20.00 

— 

Lead 

0.25 

0.58 

1.20 

0.05 

0.14 

0.26 

55 

Antimony 

0.40 

0.41 

0.42 

0.12 

0.15 

0.20 

63 

Selenium 

0.50 

0.50 

0.50 

0.50 

0.50 

0.50 

— 

Zinc 

6.4 

26.2 

34.0 

1.30 

3.53 

8.70 

70 


63 



























ied particle sizes vs removal rates. Most have concentrated 
on urban mechanical wastewater treatment systems. 
Odegaard (1987) noted that except for nickel 50 to 75% of 
the incoming metals (zinc, copper, chromium, lead, and 
cadmium) in the wastewater were associated with the TSS. 
Hannah, et al, (1986) showed that facultative lagoons re¬ 
moved 40 to 80% of metals, including nickel. The sum¬ 
mary of these and other studies is that most metals, with 
the exception of nickel, boron, selenium, and arsenic, tend 
to associate with removable solids fractions. Gearheart and 
Finney (1996) evaluated particle size removals of oxida¬ 
tion pond effluent in a FWS wetland (see Table 4-3). Their 
results show that the settleable solids (> 100 &m) portion 
of BOD, COD and TSS are essentially completely removed, 
the supracolloidal 1 to 100 &m) fraction is 80 to 90% re¬ 
moved, while the remaining (<1&m) fractions are less im¬ 
pacted. 

4.1.9 Stochastic Variability 

Free water surface (FWS) treatment wetlands demon¬ 
strate the same type of water quality variability typical of 
other complex biological treatment processes. While inlet 
concentration pulses are frequently dampened through the 
long hydraulic and solids retention times of the treatment 
wetland, there is always significant spatial and temporal 
variability in wetland water pollutant concentrations. The 
stochastic character of rainfall and the periodicity and sea¬ 
sonal fluctuation in ET contribute to much of this variability 
in the concentrations in wetland effluents. Better design 
and operational factors could reduce some of the variation 
seen in systems to date. Each site and its unique climatol¬ 
ogy require the designer to consider the effect these vari¬ 
ables will have on the sizing, depth, and configuration of 
the system. 

4.2 Wetland Hydrology 

The hydrology of FWS wetlands is considered by many 
to be the most important factor in maintaining wetland struc¬ 
ture and function, determining species composition, and 


Table 4-3. Fractional Distribution of BOD, COD and TSS in the 

Oxidation Pond Effluent and Effluent from Marsh Cell 5 
(Gearheart and Finney, 1996) 

BOD COD TSS 



mg/L 

% 

mg/L 

% 

mg/L 

% 

Oxidation Pond 

Fraction 

Total 

27.5 

100 

80 

100 

31.0 

100 

Settleable 

3.7 

13 

5 

6 

5.8 

19 

Supracolloidal 

13.7 

50 

23 

29 

25.2 

81 

Colloidal-soluble 

10.1 

37 

52 

65 

— 

— 

Marsh Fraction 

Total 

4.8 

100 

50 

100 

2.3 

100 

Settleable 

0 

0 

0 

0 

0.3 

13 

Supracolloidal 

1.2 

24 

4 

8 

2.0 

87 

Colloidal-soluble 

3.6 

76 

46 

92 

— 

— 


developing a successful wetlands project (Mitsch and 
Gosselink, 1993). The following section describes the most 
common method for characterizing wetland hydrology: the 
development of a wetland water balance. 

4.2.1 Wetland Water Balance 

The wetland water balance quantifies the hydrologic 
balance between inflows, outflows, and internal gains and 
losses of water to a wetland, in relation to the wetland vol¬ 
ume or storage capacity. The sources of water to a FWS 
constructed wetland are wastewater inflow and precipita¬ 
tion, snowmelt and direct runoff from the wetland catch¬ 
ment (i.e. berms and roads). Water losses from a FWS 
constructed wetland occur through the outlet, evapotrans- 
piration, infiltration, and bank storage (wicking). A thorough 
understanding of the dynamic nature of the wetland water 
balance, and how this balance affects pollutants, is useful 
in the planning and design of FWS constructed wetlands. 

An overall wetland water balance is the first step in de¬ 
signing a FWS constructed wetland, and should be com¬ 
pleted prior to the actual design steps described later in 
this chapter. At a minimum, a detailed monthly or seasonal 
water balance, which considers all potential water losses 
and gains, should be conducted for any proposed FWS 
constructed wetland. An annual water balance may miss 
important seasonal wetland water gains or losses, such 
as heavy periods of winter precipitation or high summer 
evapotranspiration rates, which can affect FWS constructed 
wetland pollutant effluent concentrations. Water balances 
performed over shorter time periods than monthly will cap¬ 
ture additional information about the dynamics of a wet¬ 
lands hydrology, but the increased cost of data acquisition 
will not generally be justified. 

The wetland water balance for a FWS constructed wet¬ 
land can be expressed in generic units (L=length; T=time) 
as: 

——— = Q 0 + Q f + Q sm -Q b - Q e + (P+ET+I)A W (4-1) 

dt 

where: 

A w = wetland water surface area (L 2 ), 

ET = evapotranspiration rate (L/T), 

I = infiltration to groundwater (L/T), 

P = precipitation rate (L/T), 

Q b = berm loss rate (L 3 /T), 

Q c = catchment runoff rate (L 3 /T), 

Q o = wastewater inflow rate (L 3 /T), 

Q e = wetland outflow rate (L 3 /T), 

Q sm = snowmelt rate (L 3 /T), 
t = time (T), and 

V w = water volume or storage in wetland (L 3 ). 

The impact of wet weather and snowmelt on the waste- 
water inflow (Q o ) is external to the water balance. 

Some of the terms in Equation 4-1 may be deemed in¬ 
significant and can be neglected, simplifying calculations 
and data collection requirements. For example, ground- 


64 










water infiltration (I) and berm losses (Q b ) can be neglected 
if the wetland is lined with an impermeable barrier, and the 
snowmelt (Q sm ) is only important in certain locations. 

4.2.2 Wastewater Inflow 

The daily wastewater inflow flow rate (Q o ) will almost 
always be the primary inflow into a FWS constructed wet¬ 
land. If the FWS wetland is being added to an existing 
wastewater treatment process, wastewater flow rates may 
already be measured. If the wastewater flow rates and 
variability are not known, they can be estimated using con¬ 
ventional engineering methods. Examples of variable 
wastewater flows include seasonal peaks from vacation 
communities and seasonal high infiltration and inflow rates 
into collection systems. 

4.2.3 Precipitation, Snowmelt, and 
Catchment Runoff 

Depending on the time period of the water balance, daily, 
monthly or seasonal precipitation and snowmelt data may 
be required. Precipitation inflows into a wetland come from 
direct precipitation (P) onto the wetland surface area and 
runoff from the wetland catchment (Q c ). Snowmelt 
(Q )accounts for the amount of water entering the wet- 
lana from melting snow from the wetland catchment. The 
effects of precipitation on the wetland water balance are 
normally significant, while snowmelt can be seasonably 
significant only in certain climates. Q c is a significant factor 
only in the rarest of circumstances. c 

4.2.4 Wastewater Outflow 

The wastewater outflow (Q e ) corresponds to the amount 
of treated wastewater leaving the FWS constructed wet¬ 
land over a specified time period. Wastewater outflow re¬ 
flects the balance between inflows, additional water gains 
and losses, and the change in storage of the FWS con¬ 
structed wetland. 

4.2.5 E vapotranspiration 

Wetland evapotranspiration (ET) is the combined water 
loss due to evaporation from the water surface and tran¬ 
spiration from wetland vegetation. The loss of water from 
ET affects the wetland in two ways. It increases the hy¬ 
draulic retention time by removing water, and can concen¬ 
trate certain pollutants, especially conservative dissolved 
constituents. For non-conservative constituents, such as 
BOD, an increase in the hydraulic retention time may pro¬ 
vide a modified removal rate which can either partially off¬ 
set or enhance the concentrating effects of ET. 

Specific ET rates have proven difficult to accurately 
measure in FWS wetlands. As a consequence, it is com¬ 
mon practice in wetland design to assume that wetland 
ET rates are equivalent to some percentage of open water 
or pan evaporation rates. Kadlec and Knight (1996) rec¬ 
ommend that ET be assumed equal to 70 to 80% of Class 
A pan evaporation in fully vegetated FWS systems. Reed, 
et al, (1995) suggest 80% of the pan rate. Since ET rates 


in FWS systems may vary from those in open waters to 
those in fully vegetated zones, an overall average rate may 
be useful. A rate of 70 to 75% of the pan rate is a reason¬ 
able assumption since the two are not significantly differ¬ 
ent. Maximum ET rates have been found in smaller wet¬ 
lands or wetland test cells with small area to perimeter 
ratios (Gearheart, et al, 1993). ET rates of up to 5 mm/d 
are found in the southern U.S., and Qe may approach zero 
during these periods (EPA, 1999). 

4.2.6 Infiltration and Berm Losses 

Infiltration (I) is the loss of water that occurs into the bot¬ 
tom soils or berms of a FWS constructed wetland. If 
present, infiltration decreases the outlet flow rate, effec¬ 
tively increasing the water retention time and increasing 
the potential for constituent removal. Constituent reduc¬ 
tion may be further improved by the loss of soluble pollut¬ 
ants into the soil as the water infiltrates. Infiltration tends 
to reduce with time as clogging of soil pores progresses 
(Middlebrooks, et al, 1982). If the FWS constructed wet¬ 
land is lined with some type of impermeable barrier, infil¬ 
tration can be neglected in the water balance. If not, siting 
on more permeable soils might endanger ground water 
quality. 

4.2.7 Wetland Volume 

The outlet in a FWS constructed wetland generally con¬ 
sists of a control structure that can regulate water depths 
in the wetland. Increasing or decreasing water levels 
changes the wetland volume, which influences the water 
balance by providing more or less storage capacity. The 
wetland volume (V w ) or storage capacity directly influences 
the time required for the wastewater to pass through the 
wetland. Water storage capacity can be increased to off¬ 
set the effects of high seasonal precipitation or evapotrans¬ 
piration. Since FWS constructed wetlands have a continu¬ 
ous wastewater inflow and some form of outlet water level 
control, water surface elevations do not change signifi¬ 
cantly, unless the wetland operation and maintenance 
schedule dictates water level fluctuations. The type of 
emergent aquatic plants in each region of an FWS is pri¬ 
marily determined by the depth of the FWS in that zone. 
For the most part 1.2m (4 feet) is considered the maxi¬ 
mum seasonal water depth for fully vegetated sections of 
the wetland. Normal operating depths vary from 0.5 to 
0.75m ( 1.7 to 2.5 feet) depending on the types of plants 
and types of physical substrate. 

4.3 Wetland Hydraulics 

From a design perspective, wetland hydraulics defines 
the movement of water through a FWS constructed wet¬ 
land. A FWS constructed wetland with poor hydraulic de¬ 
sign can be problematic in terms of effluent water quality, 
odors, and vector nuisances. This section first defines some 
basic wetland hydraulics terms, and then briefly summa¬ 
rizes basic wetland hydraulic principles. 


65 



4.3.1 Wetland Hydraulics Terminology 
and Definitions 

4.3.1.1 Water Depth 

Water depth is an important physical measure for the 
design, analysis, and operation and maintenance of FWS 
constructed wetlands. The ability to vary water depth in a 
FWS constructed wetland is one operational control avail¬ 
able to operators to manipulate wetland performance. 

The actual water depth at all locations in a FWS con¬ 
structed wetland will generally not be known with a high 
degree of accuracy due to basin bottom irregularities. In 
addition, the water depth in the wetland may decrease due 
to the buildup of peat from the deposition of detritus and 
settled solids buildup. Increasing the water depth by chang¬ 
ing the outlet weir elevation can help offset decreases in 
water depth. This slow-rate change progresses with time 
from the inlet toward the outlet. Detrital plant material builds 
up on and under the water surface of any vegetated zone, 
especially the initial vegetated zone. Wetlands operating 
for 15 years have documented a 0.08 -0.12m (3 to 5 inch) 
depth change due to plant detritus along the initial veg¬ 
etated zone in an FWS wetland. The largest accumulation 
was in the inlet region (Kadlecik, 1996). 

Estimated operating water depths for FWS constructed 
wetlands in the NADB (1993) have ranged from approxi¬ 
mately 0.1 to over 2.0m (0.3 to 6+ feet) with typical depths 
of 0.15 - 0.60m (0.5 to 2 feet). Operating depths are gen¬ 
erally different for the area with emergent plants (0.6 m) 
than those areas with submergent plants (1.2m). Since 
most of the NADB systems were designed to be fully veg¬ 
etated, these depths are less than one would expect in the 
future. For some calculations the average water depth (h) 
can be used, as it represents the average water depth over 
the total wetland surface area (A w ). 

4.3.1.2 Volume 

The volume (Vw) of a FWS constructed wetland is the 
potential quantity of water (neglecting vegetation, litter and 
peat) that could be stored in the wetland basin. The wet¬ 
land water volume can be determined by multiplying aver¬ 
age water depth (h) by area (AJ: 

V w = (AJ(h) (4-2) 

4.3.1.3 Wetland Porosity or Void Fraction 

In a FWS wetland, the vegetation, settled solids, litter 
and peat occupy a portion of the water column, thereby 
reducing the space available for water. The porosity of a 
wetland (e), or void fraction, is the fraction of the total vol¬ 
ume available through which water can flow. Wetland po¬ 
rosity has proven difficult to accurately measure in the field. 
As a result, wetland porosity values listed in the literature 
are highly variable. For example, Reed, et al (1995) and 
Crites and Tchobanoglous (1996) suggest wetland poros¬ 
ity values ranging from 0.65 to 0.75 for fully vegetated 
wetlands, for dense to less-mature wetlands, respectively. 
Kadlec and Knight (1996) report that average wetland po¬ 


rosity values are usually greater than 0.95, and e = 1.0 
can be used as a good approximation. Gearheart (1997) 
found porosity values in the range of 0.75 in dense mature 
portions of the Areata wetland. For hydrological design, an 
average porosity value should be used which is based on 
the areal percent of open water zones (non-emergent veg¬ 
etation) to vegetated zones. For example, a wetland with 
50% open water (e =1.0) and 50% emergent vegetation 
(e = 0.75), would have an average e = 0.875. 

The overall effects of decreasing porosity are to reduce 
the wetland volume available for water, which reduces the 
retention time of water within the wetland, and to increase 
flocculation of colloidal material which improves removal 
by sedimentation. It is recommended that a porosity value 
of 0.65 to 0.75 for fully vegetated zones be used in FWS 
constructed wetland design calculations, with lower val¬ 
ues for the most densely vegetated areas. A value of e = 
1.0 should be used for wetland open water zones. The 
use of conservative average porosity values provides a 
factor of safety, and results in a more conservative design. 

4.3.1.4 Average Wastewater Flow 

The average wastewater flow accounts for the effects of 
water gains and losses (precipitation, evapotranspiration 
and infiltration) that occur in a FWS constructed wetland. 
Defining Q o as the FWS influent flow rate and Q e as the 
FWS effluent flow rate, the average wastewater flow rate 
is expressed as: 

n _ Qo + Qe 

Vave 2 (4-3) 

If actual wastewater inflow and outflow are known, these 
values can be used in Equation 4-3. If only one of these 
flows has been measured, a water balance can be con¬ 
ducted to determine the other. If neither are known, a wa¬ 
ter balance can be useful to show the relationship between 
the two under the extreme circumstances of operation. 

4.3.1.5 Hydraulic Retention Time 

The nominal hydraulic retention time (HRT) is defined 
as the ratio of the useable wetland water volume to the 
average flow rate (Q ave ). The theoretical hydraulic reten¬ 
tion time as t can be calculated as: 

t-(VJ(e)/Q„ (4-4) 

The flow rate used in the hydraulic retention time calcu¬ 
lation can be the average wetland flow (Q ave ) or the maxi¬ 
mum or minimum flows, depending on the a purpose of the 
calculation. 

4.3.1.6 Hydraulic Loading Rate 

The hydraulic loading rate (q) is the volumetric flow rate 
divided by the wetland surface area and represents the 
depth of water distributed to the wetland surface over a 
specified time interval. The hydraulic loading rate can be 
written as: 

q = Q„ / \ (4-5) 


66 




where q has units of L/T. Generally the hydraulic load¬ 
ing rate is determined using the wastewater inflow (Q o ). 

4.3.2 Water Conveyance 

Water conveyance in FWS wetlands is hydraulically com¬ 
plex, varying in both space and time due to wetland veg¬ 
etation and litter, changing inflow conditions, and the sto¬ 
chastic nature of hydrologic events. 

When designing a constructed wetland, it is necessary 
to understand how water moves through the wetland, and 
how this water movement influences various design con¬ 
siderations. 

4.3.2.1 Ideal versus Actual Flow in a FWS 
Constructed Wetland 

Though plug flow is generally assumed for the purposes 
of FWS constructed wetland design, actual wetland flow 
hydraulics do not follow an ideal plug flow model. The de¬ 
viation from plug flow of an existing FWS constructed wet¬ 
land can be determined through the use of tracer tests. 
One result of a tracer test is the determination of the aver¬ 
age tracer retention time, which is defined as the centroid 
of the response curve, as shown in Figure 4-11. The aver¬ 
age tracer retention time is equal to the active water vol¬ 
ume (V w ) (e) divided by the average volumetric flow rate 
(Q ave ), and thus represents a direct measure of actual re¬ 
tention time. Results from some tracer studies have shown 
that the hydraulic characteristics of a FWS constructed 
wetland can be approximated by a series of 4 to 6 equally 


sized complete mix reactors (Kadlec and Knight, 1996; and 
Crites and Tchobanoglous, 1998). In other studies, the 
complete mix reactor model has resulted in a poor fit to 
the data, and other models have been more successful. 
Figure 4-11 shows the observed tracer concentration from 
one wetland cell of the Sacramento Regional Wastewater 
Treatment Plant Demonstration Wetlands Project com¬ 
pared to the predicted tracer concentrations using the fi¬ 
nite state model first suggested by Hovorka (1961). The 
finite stage model integrates components of completely 
mixed, plug flow, and off line storage addition/feedback 
into one hydraulic model. Coefficients are unique for each 
geometry, planting pattern, etc. For any given site with 
appropriate data the finite stage model gives the best fit of 
the tracer data. This method was first applied to FWS sys¬ 
tems at the Areata pilot studies (Gearheart, et al, 1983) 
and has subsequently been applied to the Sacramento 
system (Dombeck, 1998). The value of multiple cells and 
periodic open-water zones have been recognized for mini¬ 
mizing short-circuiting by numerous authors. 

4.3.2.2 Hydraulic Gradient in a FWS 
Constructed Wetland 

For FWS constructed wetlands, some assessment of the 
energy loss or head loss from inlet to outlet is necessary 
to ensure that the wetland is designed to handle all poten¬ 
tial flows without creating significant backwater problems, 
such as flooding the inlet structures or overtopping berms. 
It has historically been assumed that Manning's equation, 
which defines flow in open channels, can be adapted to 



Figure 4-11. Tracer response curve for Sacramento Regional Wastewater Treatment Plant Demonstration Wetlands Project Cell 7 (SERSD, 1998). 


67 











estimate head loss in FWS wetlands. By assuming that 
the submerged wetland vegetation, peat and litter provides 
more frictional resistance to flow than the wetland bottom 
and sides, Manning's equation has been adapted as fol¬ 
lows: 


where: 

v = average flow velocity (L/T), 
n = Manning’s resistance coefficient (T/L 1/3 ), 
h = average wetland depth (L), and 
S = hydraulic gradient or slope of water surface (L/L). 

In the above equation, the average wetland depth and 
water surface slope is fairly easily estimated, and the av¬ 
erage velocity (v) is defined as the average flow (Q ) di¬ 
vided by the available average cross sectional area (A v )(e), 
or (Width)(depth)(e). The determination of Manning's re¬ 
sistance coefficient (n) is not as straightforward. In wet¬ 
lands, the vegetation and litter providing resistance to wa¬ 
ter flow is distributed throughout the water column, with 
settled particles and detritus on the bottom and a thicker 
thatch level at the top. Thus, n should be a function of the 
water depth as well as the resistance of specific surfaces. 
Measurements of n in operating wetlands range from ap¬ 
proximately 0.3 to 1.1 s/m 1/3 with the higher numbers cor¬ 
responding to water depths less than 0.2 m (Kadlec and 
Knight, 1996). Reed, et al, (1995) use an equation to esti¬ 
mate n at 0.2m depth to vary from 1 to 14 s/m 1/3 . Linsley, 
et.al., (1982) offer a series of values for n from0.024 to 
0.112. A default value of 0.1 to 0.5 is suggested for those 
wishing to pursue this issue. Atypical solution provided in 
Crites and Tchobanoglous (1996) is a slope of 1 in 10,000, 
or 1cm in 100 meters. Since multiple cells are recom¬ 
mended as good design practice to minimize short-circuit¬ 
ing and to maximize treatment performance, the above 
analysis is superfluous for most applications where aspect 
ratios (length/width) are within suggested limits of 3:1 to 
5:1, or even larger. 

4.4 Wetland System Design and Sizing 
Rationale 

4.4.11ntroduction 

As FWS constructed wetlands became recognized as a 
viable wastewater treatment process, FWS design mod¬ 
els soon followed. These models were intended to aid en¬ 
gineers/designers in the process of FWS wetland design 
and performance assessment. To date, a number of wet¬ 
land design methods have been proposed for predicting 
constituent removals in FWS wetlands. These may be 
found with explanation in Reed, et al, (1995), Kadlec and 
Knight, (1996) and Crites and Tchobanoglous, (1998). The 
design models and methods have been used to attempt to 
predict the fate of BOD, TSS, TN, NH 4 , N0 3 , TP and fecal 
coliforms in a FWS system. 

Free water surface constructed wetlands have usually 
been modeled as attached growth biological reactors, in 


which the plants and detrital material uniformly occupy the 
entire volume of the wetland. 

The current trend in wetland design modeling is the de¬ 
velopment of simple mass balance or input/output mod¬ 
els. These simplified models do not explicitly account for 
the many complex reactions that occur in a wetland, either 
in the water column or at interfaces such as the water/ 
sediment interface. Instead, all reactions are lumped into 
one overall biological reaction rate parameter that can be 
estimated from collected FWS wetland performance data. 
At this stage of wetland understanding, more complex and 
theoretical wetland models which explicitly describe the 
kinetics of known wetland processes may not be possible 
due to severe limitations in almost all of the existing wet¬ 
lands data. 

4.4.2 Existing Models 

In essence the types of models that have been used in 
FWS constructed wetland design are known as plug-flow- 
reactor (PFR) models.. One assumes horizontally based (lin¬ 
ear) kinetics (Reed, et al,1995; Crites and Tchobanoglous, 
1998), while the other assumes vertical (areal) kinetics 
(Kadlec and Knight, 1996). Several varieties of these mod¬ 
els exist. Some assume average kinetic rate constants, while 
others assume retarded kinetic rate constants. All provide a 
list of effluent background concentrations below which an 
FWS cannot dependably attain and specific default values 
for temperature adjustments to correct kinetic rate constants. 
Some suggest monthly multipliers for average computed 
design performance. Some include safety factors within the 
equation while others apply them to the model result. All as¬ 
sume first-order biological kinetics, despite the fact that the 
initial fully vegetated treatment zone is anaerobic, and none 
of these models can account for a sequencing, i.e., fully veg¬ 
etated and open-water zones in sequence, design which is 
recommended herein for better performance. Recently, one 
of the primary model creators has also noted the inadequacy 
of these models (Kadlec, 2000). Readers are referred to 
Kadlec and Knight (1996), Reed, et al, (1995), and Crites 
and Tchobanoglous (1998) for details. For the purpose of 
this manual, i.e., providing secondary (BOD=SS =30mg/l) 
and advanced secondary treatment of municipal wastewa¬ 
ters, none of these equations alone are able to accurately 
predict the performance of a multi-zone FWS constructed 
wetland. Even if they could be calibrated “to fit” a specific set 
of data their non-deterministic basis belies their ability to fit 
other circumstances of operation. 

4.4.3 Areal Loading Rates 

The areal loading rate method specifies a maximum load¬ 
ing rate per unit area for a given constituent. These meth¬ 
ods are common in the design of oxidation ponds and land 
treatment systems. Areal loading rates can be used to give 
both planning level and final design sizing estimates for 
FWS systems from projected pollutant mass loads. For 
example, knowing the areal BOD loading rate, the expected 
BOD effluent concentration can be estimated or compared 
to the long term average performance data of other well- ■ 
documented, full-scale operating systems. 


68 





In section 4.1 each pollutant was discussed based on 
areal loading vs. effluent concentration based on the 
DMDB, the TADB, specific studies and mechanistic evalu¬ 
ations of other sources of information. Areal loading does 
not always correlate to a reasonable design basis, espe¬ 
cially with regard to nutrients and pathogen removal, and 
other mechanistic explanations are necessary. However, 
if typical municipal wastewaters are to be treated which 
have total and filtered pollutant fractionation which are rea¬ 
sonably consistent from site to site, a rational design ap¬ 
proach can be deduced for those parameters which can 
be removed during the enhanced flocculation/sedimenta¬ 
tion which occurs in the initial fully vegetated zone of a 
FWS constructed wetland. Therefore, based on Figures 
4-1 and 4-4 the following areal loadings can be employed 
for this initial zone (zone 1) of the FWS : 

Parameter Zone 1 Areal Loading Effluent Concentration 

BOD 40 kg/ha-d 30 mg/L 

TSS 30 kg/ha-d 30 mg/L 

The relative areal loadings imply that unless the pre¬ 
treatment process were to have a BOD concentration of 
greater than 1.3 times the TSS concentration, the latter 
would be the critical loading rate for the fully vegetated 
zone if secondary standards are to be met by a fully veg¬ 
etated FWS system. 

If the FWS system were to have significant open areas 
between fully vegetated zones, a better effluent quality 
could be attained at areal loadings, based on the entire 
FWS system area (AJ: 

Parameter Areal Loading Effluent Concentration 

BOD 45 kg/ha-d <20 mg/L 

60 kg/ha-d 30 mg/L 

TSS 30 kg/ha-d < 20 mg/L 

50 kg/ha-d 30 mg/L 

These loadings are based on the entire system area, 
not just zone 1. Therefore, with open-water zones which 
provide aerobic transformations and removal opportuni¬ 
ties, a better effluent quality is achievable than with a fully 
vegetated FWS system. Although there are insufficient data 
at this time to eliminate the need to provide effluent disin¬ 
fection, more disinfection interferences are removed which 
would facilitate that step. Conversely, the open water zones 
would attract wildlife to a greater degree, and the impacts 
created by their activities. Similarly, the need to and the 
power required to reaerate the final effluent will at the least 
be reduced. The advantages of this design concept have 
been described by Gearheart and Finney (1996) to include 
reduced “background” BOD concentrations in the effluent 
owing to the aerobic biological removals in the open-water 
zones. As with the fully vegetated systems, the TSS areal 
loading is more critical. With more quality data these limit¬ 
ing loadings could be shown to be conservative, especially 
the BOD loading for attaining secondary effluent standards 
with open-water FWS systems. 


4.5 Design 

4.5.1 Design Sizing and Performance 
Mechanisms 

If a pretreatent system already exists, the type of influ¬ 
ent characterization necessary has already been dis¬ 
cussed, but at a minimum all pollutants which are of con¬ 
cern to the NPDES permitting authority should be mea¬ 
sured as both total and filtered through a standardized glass 
fiber filter prior to analysis. Ideally, a particle-size distribu¬ 
tion analysis of the type described in Crites and 
Tchobanoglous (1998) could be performed for all critical 
pollutants to aid the designer in predicting what level of 
removals of each pollutant are likely to be attained by an 
FWS or other treatment processes. If the pretreatment 
system does not exist, the designer will need to perform a 
variety of investigations as described in several engineer¬ 
ing texts (e.g., Crites and Tchobanoglous,1998; WEF, 
1998). 

A primary supposition of this manual is that a FWS con¬ 
structed wetland is most likely to treat effluent from a sta¬ 
bilization or oxidation pond or from primary-treated (settled) 
municipal wastewater. After the designer determines overall 
size of the FWS system from these BOD and TSS areal 
loading rates, he or she can return to evaluate the fate of 
other constituents. 

If the total and filtered analyses are available, it is a rea¬ 
sonable approximation to assume that the filtered analy¬ 
sis represents a rough approximation of effluent quality 
attainable from treatment zone 1 (fully vegetated zone) 
given that the filter pores are generally a bit larger than the 
specific particle sizes indicated for the “colloidal/soluble” 
fraction in Table 4-3. Internal loads in the soluble form 
should also be added to this fraction in estimation of zone 
1 effluent(see Figure 4-2 and 4-3). For more stringent ef¬ 
fluent requirements than those cited above for BOD and 
TSS, the designer should look at alternative polishing pro¬ 
cesses such as land treatment or slow sand filtration. 

While a few physical and chemical processes occur uni¬ 
formly over the entire wetland volume, many of the most 
important treatment processes occur in a sequential man¬ 
ner and the wetland must be designed to accommodate 
this characteristic. For example, TSS removal and removal 
of associated BOD, Org N and P, metals, etc., occur in the 
initial portion of the cell, while the subsequent zones can 
impact certain soluble constituents. Given sufficient dis¬ 
solved oxygen in open (unvegetated) areas, soluble BOD 
removal and nitrification of ammonia can occur. If insuffi¬ 
cient oxygen is present, soluble BOD is very slowly re¬ 
moved by anaerobic processes. Wetland design must also 
consider the background level, or expected lower limit, of 
water quality constituents in the FWS wetland effluent (see 
Table 4-4). Particulate and soluble constituents are inter¬ 
nally produced as a part of the normal decomposition and 
treatment processes occurring in a constructed wetland. 
Wildlife contribute fecal coliform and additional organic 
compounds. During periods of intense activity, wildlife also 


69 







stir up settled solids contributing to an increase in turbid¬ 
ity, TSS, and BOD. Table 4-4 shows the typical background 
levels for the constituents of interest recommended for 
users of this document. Designs requiring effluent quality 
close to the values in Table 4-4 must be aware of the natu¬ 
ral fluctuations about the mean values, as shown in Figure 
4-12. For more details on the numerical values in the table, 
the reader is urged to refer to Reed, et al, (1995), Kadlec 
and Knight, 1996, and Gearheart, 1992. 

A similar approach to the one suggested here for de¬ 
signing FWS wetlands, referred to as the "sequential 
model", has been developed by Gearheart and Finney 
(1999). The overall approach of the model is to consider 
the dominant physical and biological processes respon¬ 
sible for determining effluent quality from each distinctive 
area or zone of the constructed wetland and allow the de¬ 
signer to specify areal requirements and wetland depth for 
each of these specific functions. This methodology recog¬ 
nizes that while some of the constituent transformations 
and removal mechanisms are to some degree occurring 
simultaneously throughout the wetland, the majority of the 
removal occurs in a sequential fashion, with one process 
or mechanism providing the products for the next process 
or mechanism. The total area required for treatment is then 
a sum of each of the zones required to reach a specific 
effluent objective. This approach allows the designer to 
sequentially determine the range of effluent characteris¬ 
tics which are attainable in a given definable zone before 
entering a subsequent reactor (zone) which has known 
treatment capabilities. 


Table 4-4. Background Concentrations of Water Quality Constituents 
of Concern in FWS Constructed Wetlands 


Range Typical 

Parameter (mg/L) (mg/L) Factors governing 


TSS 

2-5 

3 

Plant types, plant coverage, climate, 
wildlife activity 

BOD' 

2-8 

5 

Plant types, plant coverage, plant 
density, climate, wildlife activity 

BOD 2 

5-12 

10 

Plant types, plant coverage, plant 
density, climate 

TN 

1 -3 

2 

Plant types, plant coverage, climate, 
oxic/anoxic conditions 

NFF-N 

4 

0.2- 1.5 

1 

Plant types, plant coverage, climate, 
oxic/anoxic conditions 

TP 

soil 

0.1 -0.5 

0.3 

Plant types, plant coverage, climate, 
type 

FC 3 

50 - 5000 

200 

Plant types, plant coverage, climate, 
wildlife activity 


'FWS with open water and submergent and floating aquatic macro¬ 
phytes. 

2 Fully vegetated with emergent macrophytes and with a minimum of 
open water. 

2 Measured in cfu/100 ml 


The sequential model approach recognizes that all the 
treatment objectives beyond secondary require a minimum 
of three general wetland "compartments" (see Figure 4- 
13): (1) an initial compartment where the bulk of the floc¬ 
culation and sedimentation will occur, (2) an aerobic com¬ 
partment where soluble BOD reduction and nitrification can 
occur, and (3) a vegetated polishing compartment where 
further reductions in TSS and associated constituents and 
nitrogen (via denitrification) can occur. Permanent phos¬ 
phorus removal in wetlands is generally small and is largely 
the result of phosphorus adsorption to solids and plant 
detritus. Sedimentation and pathogen reduction are related 
to detention time in zone 1, to retention time and tempera¬ 
ture in zone 2, and to retention time in zone 3. As noted 
earlier, the notion of "compartments" is artificial as the treat¬ 
ment processes overlap in time and space, and no spe¬ 
cific physical compartment is necessarily implied. However, 
separation of an FWS into a series of single-function zones 
(cells) with individual outlet controls is not an unattractive 
concept. 

A rational overview of the FWS system is depicted in 
Figure 4-14. It illustrates that the primary mechanisms in 
zone 1, which is fully vegetated and anaerobic throughout 
its depth during the growing season, are sedimentation 
and flocculation, as determined by transect measurements 
of dissolved oxygen and pollutant concentrations. Any ex¬ 
tension of the HRT in zone 1 beyond approximately 2 days 
at Qmax would be essentially wasteful since the anaero¬ 
bic conditions will not result in any significant further re¬ 
moval of soluble constituents and flocculation sedimenta¬ 
tion has been effectively completed. The TSS and associ¬ 
ated constituents (particulate BOD, organic nitrogen and 
phosphorous, metals and certain semivolatile organic com¬ 
pounds) have also reached this same status. Volatile or¬ 
ganics are likely to be removed from the wastewater dur¬ 
ing the collection or oxidation pond treatment processes 
(Hannah, et al, 1986), while most semivolatiles are removed 
with the solids in the oxidation pond or in zone 1 of the 
FWS system. 

For many years it has been recognized that effluent floc¬ 
culation is primarily a function of energy input from either 
external sources or internal hydrodynamic forces, and that 
reduced Reynolds’ Numbers (Re) induce optimal sedimen¬ 
tation of particles. Over the past several decades this phe¬ 
nomena has been applied in the development of hydrody¬ 
namic devices which accomplish excellent flocculation and/ 
or sedimentation without moving parts, such as pipe mix¬ 
ers and flocculators, tube and plate settlers, and pebble 
bed and wedgewire outlet devices for clairifers. Flow 
through the emerging vegetation is extremely tortuous and 
is accompanied by a very small hydraulic radius. The 
Reynolds Number (Re) is a direct function of the hydraulic 
radius (diameter, if the path were round (as in a pipe). If 
the Re falls in a range which corresponds to laminar flow, 
sedimentation is maximized. Re is several thousand in large 
basins, and even larger in non-vegetated ponds. No direct 
measurements of Re or laminar flow have been made at 
the time of this writing, but analogous results from studies 


70 






150 


O) 

S 

Q 

o 

CO 


—Q— 

Mean 

-0- 

Median 

—A— 

Minimum 

—©— 

Maximum 



70 


Distance from Influent (m) 


Figure 4-12. Mean, median, minimum and maximum transect BOD 5 data for Areata Pilot Cell 8 


Floating and 

Inlet Settling Zone Emergent Plants 


Submerged 
Growth Plants 


Floating and Emergent 
Plants 



Zone 1 

Fully Vegetated 
D.O. (-) 

H < 0.75 m 


Zone 2 

Open-Water Surface 
D.O. (+) 

FI > 1.2 m 


Zone 3 

Fully Vegetated 
D.O. (-) 

H < 0.75 m 


Figure 4-13. Elements of a free water surface (FWS) constructed wetland 


71 














































Zone 1 


Zone 2 


Zone 3 



Figure 4-14. Generic removal of pollutants in 3-zone FWS system 


of tube settlers and particle-size removals support this 
theory, given the large amount of wetted surface available. 
(Sparham, 1970). This concept also supports the use of 
fully vegetated areas immediately preceding outlet weirs. 

In zone 2, which is primarily open-water, the natural 
reaeration processes are supplemented by submerged 
macrophytes during daylight periods to elevate dissolved 
oxygen in order to oxidize carbonaceous compounds 
(BOD) to sufficiently low levels to facilitate nitrification of 
the NH 4 -N to N0 3 -N. These processes require large 
amounts of oxygen and time in a passive system (no me¬ 
chanical assistance). The maximum HRT in zone 2 is gen¬ 
erally limited to about 2 to 3 days before unwanted algal 
blooms occur. Therefore, more than one open zone may 
be required to complete these reactions. If so, the result 
would be a five (or more) zone design since each open 
zone would be followed by a fully vegetated zone. The 
reactions in zone 2 are essentially the same as in a facul¬ 
tative lagoon. Therefore, the equations which apply to those 
systems might offer reasonable approximations to the rate 
of transformations occurring in this open-water zone. There¬ 
fore, the first-order Marais and Shaw (1961) equation for 


fecal coliform dieoff could be applied as an approximation, 
along with its temperature dependancy: 


Co (l + tK p ) N 

where: Co = influent FC concentration, cfu/100 ml 
Ce = effluent FC concentration, cfu/100 ml 
N = number of open-water zones in the 
FWS 

t = HRT (T) 

K = fecal coliform removal rate constant 

(T 1 ) 

= 2.6 ( 1 . 19) T - 20 ( 4 - 8 ) 

where: T = temperature, °C 

BOD removal in the open-water zone should also follow 
existing equations such as (Crites and Tchobanoglous, 
1998); 

— = -!- w (4-9) 

Co (l + tK p ) N 


72 


























where: 

C = BOD, mg/L 

Kb = specific BODs removal rate constant 

(T 1 ) 

Kb = 0.15 (1.04) 1 ' 20 (4-10) 

Therefore, in analyzing Figure 4-14 the downward stope 
in FC and BOD in zone 2 can be approximated through 
the above equations, without considering offsets from wild¬ 
life. As noted previously, the nitrifying bacteria can prolif¬ 
erate and convert ammonia-nitrogen to nitrate(N0 3 -N) and 
will be the primary nitrogen transformation role of zone 2. 
However, the carbonaceous BOD must be low enough to 
allow these reactions to occur. In rotating biological 
contactors this concentration of BOD is about 15 mg/I 
(USEPA, 1993). As noted by Gearheart (1992) increasing 
the size of the open-water zone generally increases dis¬ 
solved oxygen, pH, and N0 3 -N, while decreasing soluble 
BOD and ammonium. 

U.K.’s Department of Environment has studied lagoon 
systems treating similar quality influent to that of zone 2. 
They have noted that algal growth generally starts to oc¬ 
cur between days 2 and 3 (UK, 1973). Algal growth can 
raise pH, interfere with FC kill and the growth of submerged 
plants, increase NH 3 -N volatilization, and induce phospho¬ 
rus precipitation. Also, the additional biomass and precipi¬ 
tates that must be removed in zone 3 will add to the inter¬ 
nal loading on the FWS system. The primary goal of the 
open-water zone is to provide dissolved oxygen to remove 
BOD and convert NH 4 -N to N0 3 -N. Therefore, the optimium 
sizing of this zone would be an HRT of 2 to 3 days. Assum¬ 
ing a Q max /Q ave of 2, the designer might choose an HRT of 
2 days at x Q max or an HRT of 3 days at Q ave . Climate would 
likely be the final criterion, with the larger size favored in 
northern areas and the smaller in southern ones. 

The third zone is fully vegetated like zone 1 and has a 
similar function. Zone 3, like zone 1 is also capable of deni¬ 
trification if the influent flow contains N0 3 -N. Where oxida¬ 
tion pond pretreatment of municipal wastewaters is em¬ 
ployed, zone 1 of the FWS system is not generally required 
to denitrify, but zone 3 will if zone 2 induces nitrification. 
The primary energy source for successful denitrification is 
the release of organic substrates from the detritus from 
decaying plants. However, partially digested, previously 
removed organics may also be available. Denitrifying bac¬ 
teria perform only under anaerobic conditions and best 
when attached to large surface areas, e.g., plants. Denitri¬ 
fication, like nitrification, is temperature-sensitive. Nitrifi¬ 
cation and denitrification are greatly impaired when water 
temperatures are reduced below 10°C. Gearheart (1992) 
showed total inorganic nitrogen in the Areata Marsh to be 
reduced from 25 to 5 mg/L. In 1995 he demonstrated pilot- 
scale removal of N0 3 -N from 130 mg/L to 6 mg/L using no 
supplemental carbon sources in 80 hours at 15°C. The 
primary limitation in a three-zone FWS system designed 


to remove nitrogen is the rate of nitrification in the open- 
water zone. If the open-water zone succeeds in nitrifying 
the NH 4 -N, the system should be able to denitrify it. Reed, 
et al, (1995) indicate that denitrification should require less 
than one day hydraulic retention time (HRT) for denitrifica¬ 
tion from municipal wastewater concentrations to an efflu¬ 
ent requirement of < 10 mg/L. Kadlec and Knight (1996) 
found that 1 to 2 days should suffice to reach 90% N0 3 -N 
removal. Therefore the previously- stated requirement for 
zone 3 (HRT of 2 days) should meet this retention require¬ 
ment and ensure significant denitrification. WEF Manual 
of Practice FD-16 (1990) indicates that the denitrification 
rate can be as high as 10 kg/ha-d. Loadings must be within 
the limits of available labile carbon to proceed at the maxi¬ 
mum rate. 

As with zone 1 there is additional, temporary nutrient (N 
and P) removal by plant uptake in zone 3, which may be 
significant at certain times during the year, while release 
of most of these nutrients occurs at other times. These 
plant effects can mask the effects of other processes which 
could be impacting the system performance at the same 
times. Unfortunately, there are insufficient data to fully quan¬ 
tify the nutrient cycle for each zone of the FWS system. 

4.5.2 Total Suspended Solids Removal 
Design Considerations 

Since prior discussion indicates that TSS removal (rather 
than BOD removal) drives the sizing process, there is a 
need to provide further discussion of the mechanisms in¬ 
volved and their implications on design. Treatment mecha¬ 
nisms which dominate in the vegetated inlet zone of a FWS 
constructed wetland volume are flocculation, sedimenta¬ 
tion and anaerobic decomposition. Discrete and flocculent 
settling occurs as the wastewater flows through the initial 
fully vegetated zone. Since the FWS was likely preceded 
by an oxidation pond where most discrete settling has oc¬ 
curred already, the enhanced settling in zonel is mostly 
due to flocculation of large supracolloidal solids in pas¬ 
sage through the emergent vegetation. The processes are 
generally not temperature dependent and occur at rela¬ 
tively high hydraulic loading rates. TSS removal rates of 
40 to 60% are common with a q of 0.06 m/day to 0.27 m/ 
day, but relative removals are more accurately determined 
by influent characteristics and the hydrodynamics of the 
initial vegetated zone. 

The majority of incoming solids are removed in this ini¬ 
tial settling volume. Hyacinth and duckweed systems are 
similar to (but not as good as) zone 1 of an FWS in the 
hydrodynamics which promote excellent flocculation and 
sedimentation. The mechanisms of the fully vegetated zone 
1 can be estimated from the use of particle size distribu¬ 
tion analysis. Generally, wastewaters have been analyzed 
in form size ranges: 

Settleable (>100 pm) 

Supracolloidal (1 to 100 |im) 

Colloidal (0.001 to 1 pm) 

Dissolved (< 0.001 pm) 


73 







Only one study has employed this approach (Gearheart 
and Finney, 1996) with oxidation ponds followed by FWS 
constructed wetland treatment. The results shown in Table 
4-3, clearly demonstrate the essentially complete removal 
of the settleable fraction (100% for BOD and 95% for TSS) 
and progressively reduced removal of the supracolloidal 
(91 % of BOD and 92% of TSS) and colloidal (66% of BOD) 
fractions. This progression runs counter to the frequently 
noted biological reaction rate vs particle size relationship 
(Levine, et. al., 1991). Therefore, the primary mechanism 
for removal of TSS and associated pollutants (BOD, or¬ 
ganic nutrients, metals and toxic organics) is not biologi¬ 
cal in nature. This would appear to be reinforced by the 
lack of dissolved oxygen, high oxygen demand, and the 
slow nature of anaerobic biological reactions which are 
the predominant biological mechanisms. 

The solids that are removed undergo incomplete anaero¬ 
bic decomposition (acidification) resulting in a release of 
nitrogen, phosphorus, and carbon in the form of volatile 
fatty acids. The amount of accumulated internal load de¬ 
pends on the length of time the water temperature stays 
below 5-10°C since this material does not undergo signifi¬ 
cant decomposition until the water temperature increases 
above this threshold value. The longer the uninterrupted 
period of less than 5-10°C, the greater the initial load and 
its effect on dissolved BOD at temperatures above this 
threshold. In most temperate North American climates, the 
release of this accumulated organic material expresses 
itself mostly in the late spring and in the early summer, 
similar to oxidation pond "spring turnover". Some of these 
impacts are noted by the comments within Figure 4-14, 
which show how some of these phenomena might impact 
removal patterns. 

Non-degradeable material is removed, accumulates and 
is compressed forming an organic layer of biologically re¬ 
calcitrant material in the sediments of zone 1. The layer is 
thicker near the influent end of the wetland and gets shal¬ 
lower in the direction of flow. This delta of accumulated 
solid material can eventually reduce the HRT and the avail¬ 
able solids storage volume of the wetland. These losses 
are also acerbated by accumulated plant detritus. These 
accumulated solids ("sludge" or "biosolids") will occasion¬ 
ally need to be removed and managed, e.g., directly land 
applied and plowed under as a soil amendment or through 
some other method as directed by the regulatory authori¬ 
ties. 

The reduction in wetland volume due to settled solids, 
living plants and plant detritus can be significant over the 
long term. The rate of accumulation of settled suspended 
solids is a function of the water temperature, mass of influ¬ 
ent TSS , the effectiveness of TSS removal, the decay 
rate of the volatile fraction of the TSS, and the settled TSS 
mass which is non-volatile. The plant detritus buildup is a 
function of the standing crop and the decay rate of the 
plant detritus. Accumulation for emergent vegetated areas 
of the Areata enhancement wetlands was measured to be 
approximately 12 mm/year of detritus on the bottom due 
to plant breakdown and 12 to 25 mm/year of litter forming 


a thatch on the surface (Kadlecik, 1996). The volume of 
the living plants, specifically the volume of the emergent 
plants, ranged from 0.005 m 3 /m 2 (low stem density, water 
depth of 0.3 m) to 0.078 m 3 /m 2 (high stem density, water 
depth of 0.75 m). This accumulation is more or less con¬ 
stant from year to year as the wetland matures. The total 
volume reduction under the initial vegetated zone can be 
estimated using a mass balance equation: 

V r = [(VJ(t) + (V d )(t)]A w (4-11) 


where: 

V r = volume reduction over period of analysis (m 3 ), 
V ss = volume reduction due to non-volatile TSS and 
non-degradable volatile TSS accumulation 
(m 3 /ha-yr), 

V d = volume reduction due to non-volatile detrital 
accumulation as A function of annual production 
(m 3 /ha-yr), 

A w = fully vegetated wetland area (ha), 
t w = period of analysis usually (years). 

The loss of volume per hectare over a ten year period 
for a 1 hectare fully vegetated FWS wetland zone with a 
depth of 0.75 m can be estimated by use of this equation. 
Based on information in Middlebrooks, et al, (1982) and 
Carre, et al, (1990) a reasonable default value for V when 
treating raw wastewater in lagoons) would be 200 to 400 
m 3 /ha-yr (2 to 4 cm/yr). Therefore, a conservative default 
value of 150 m 3 /ha-yr can be used. One hundred percent 
coverage of emergent vegetation was measured to con¬ 
tribute 120 m 3 /ha- yr of bottom detritus, and 120 m 3 /ha-yr 
of surface litter with a standing crop volume of 412 m 3 /ha. 
Substituting into the equation for a 10-year analysis yields: 

Vr =[150)(10) + (240)(10)] 1.0 = 3,900m 3 

Table 4-5 provides additional examples of wetland vol¬ 
ume loss due to TSS and plants detritus. Based on the 
actual Areata experience, it is clear that use of equation 4- 
11 is a conservative means of estimating volume reduc¬ 
tion from TSS deposition and detrital accumulation. 

Using the initial fully vegetated zone volume (V,) and 
adding the standing crop (V c ), the total loss of volume can 
be estimated by addition to be 4,312 m 3 . Since the original 
volume is area (10,000 m 2 ) times depth(0.75 m) or 7,500 
m 3 , the total loss of volume would be 4,312/10,000 or 43%. 
This corresponds to a new porosity (e) of 0.57. As noted 
earlier dense, mature stands of emergent plants are as¬ 
sumed to have a porosity of about 0.65. 

Table 4-5. Examples of Change in Wetland Volume Due to Deposition 
of Non-Degradable TSS (V ss ) and Plant Detritus (V d ) Based 
on 100% Emergent Plant Coverage (Gearheart, et al, 1998) 
Influent TSS. 


(mg/L) 

v s 

50% removal 

(m 3 /yr)ha) 

75% removal 

V d (m 3 /ha/yr) 

40 

75 

113 

240 

60 

80 

112 

150 

168 

225 

240 

240 


74 







The accumulation computed above indicate that the call 
is nearly ready for residual solids removal, as its excess 
storage capacity is essentially used up. However, the Areata 
facility for which the accumulation measurements were 
made is still performing well after 12 years (USEPA, 1999). 
The loss of volume and porosity computed by the previ¬ 
ous method is obviously conservative, but illustrates how 
one could conservatively estimate the loss of porosity in 
the initial settling zone. 

When designing the primary wetland cell treating oxida¬ 
tion pond effluent the designer should consider this cumu¬ 
lative problem by increasing the depth of the inlet zone 
(up to 1 .Om) to lengthen the period before solids removal 
would be required. The designer should also provide for 
easily accessible solids removal in this zone. There may 
be a need to harvest vegetation and related detritus to 
maintain fully vegetated and open-water areas in proper 
proportion. Such controlled harvesting may be extended 
into the fully vegetated zones to reduce the apparent loss 
of effective treatment volume and delay the need to re¬ 
move accumulated solids. 

The fully vegetated, anaerobic zone 1 of the FWS wet¬ 
land should be designed based upon the average maxi¬ 
mum monthly flow rate (G max ) to assure the potential for 
effective removal of solids during periods of high flow. To 
facilitate solids removal and handling, this initial compart¬ 
ment should be designed as at least two equally sized 
wetlands with a 0.6 to 0.9 m operating depth, which can 
be operated in parallel. This would allow taking one cell 
out of operation for maintenance work such as for solids 
removal, vegetation removal, or replanting. 

4.5.3 Design Examples 

Design Example 1 - BOD and TSS to meet secondary 
effluent requirements 

Design a FWS wetland to treat lagoon effluent to meet a 
monthly average 30 mg/I BOD and TSS discharge objec¬ 
tive. The community has a design population of 50,000 
people with an average annual design flow of 18,920 m 3 / 
day (5 MGD) (Q ave ). Use design loading factors from sec¬ 
tions 4.4 and 4.5* to meet a 30 mg/I BOD and TSS effluent 
standard. Since a single fully vegetated FWS system can 
be employed with maximum areal loading rates for these 
systems are 40 kg BOD/ha-d and 30 kg TSS/ha-d. Facul¬ 
tative lagoon effluent typically averages from 30 to 40 mg 
BOD/L and 40 to 100 mg TSS/L, with the latter being much 
more variable due to seasonal algal growth and spring and 
fall overturn periods (WEF, 1998; Middlebrooks, et al, 
1982). For this example the average FWS influent BOD is 
50 mg/I at Q ave (18,920 m 3 /d), while the average TSS is 70 
mg/L at this flow. At the maximum monthly flow (Q max ) of 2 
x Q ave the BOD is 40 mg/L and TSS is 30 mg/L. 

I! Step 1 - Apply areal loading rates(ALR) to average (Q ave ) 
and maximum monthly flow (Q^J conditions to iden¬ 
tify the critical conditions for sizing of the facility. 


ALR = Q Co/A w (4-12) 

where: 

ALR = areal loading rates: BOD = 40 kg /ha-d ;TSS 

= 30 kg /ha-d 

Q0 = incoming flow rate, in m 3 /d 
CO = influent concentration, in mg/L 
A = total area of FWS, in ha 

w 9 

which for BOD yields: 

for Q ave , A w = (18,920 m 3 /d)(1000 L/m 3 )(50 mg/L)/ (40 
kg/ha-d)(106 mg/kg) = 24 ha 

for Q max , A w = (37,840) (40)/ (40) (106) = 38 ha 

Similarly, for TSS: 

for Q ave > A w = (18,920) (70)/(30)(106) = 44 ha 
for Q max , A w = (37,840)(30)/(30)(106) = 38 ha 

Therefore, the limiting condition is the TSS loading at 
average flow conditions, where 44 hectares are required 
to meet secondary effluent standards with a fully vegetated, 
single-zone FWS system. However, it has been previously 
shown that open water zones permit higher areal loading 
rates (from section 4.4), so the sizing can be recomputed 
on that basis following the same procedure. From that 
analysis, with BOD and TSS loading rates of 60 and 50 
kg/ha-d, respectively, the critical condition is still the aver¬ 
age flow condition and the TSS areal loading, but with a 
requirement of 26 ha instead of 44 ha. 

Step 2 - Determine the theoretical HRT(days) using equa¬ 
tion 4-4, assuming h = 0.6 m and e = 0.75 in veg¬ 
etated zones (1 and 3) and h = 1.2 m and e = 1.0 in 
the open zone (2). The combined estimate is an aver¬ 
age depth of 0.8 m and an average e = 0.8. Therefore, 
the first estimate is for overall HRT, followed by indi¬ 
vidual cell estimates. 

f° r Q ave , ^ _ Vvve _ Aw h £ _ 

Qave Qave 

(26 ha)(l 0,000 m 2 / ha)(0.8 m)(0.80) 

18,920 m 3 d 

= 8.9 days 

for 9™, 1 = 4-5 days 

This last calculation implies that at the maximum monthly 
flow the overall HRT may not be adequate for the neces¬ 
sary treatment mechanisms to perform. If these relatively 
equal-sized zones are employed as a first approximation, 
there would be less than one day of theoretical HRT in 
each at this maximum flow condition. For zone 1 the mini¬ 
mum HRT at Q max should be about 2 days, making 4 days 
at Q . 

ave 

For zone 2 there is an upper limit which depends on 
climate and temperature. In this example, the concept is 


75 











that the open-water area will have an HRT exceeding that 
which is required for an algal bloom, creating a significant 
additional loading on zone 3. The time required for this 
condition varies with temperature, e.g., shorter in hotter 
climates. For most U.S. conditions a maximum HRT of 
about 2 to 3 days should avoid most blooms. On the other 
hand, the longer the HRT in zone 2, the better the reduc¬ 
tion in soluble organics, ammonia-nitrogen and fecal 
conforms. Therefore, the designer should consider isola¬ 
tion of this zone from the fully-vegetated zones that pre¬ 
cede and follow it and provide some flexibility in HRT con¬ 
trol independent of the other zones to optimize this zone’s 
treatment performance. For this exercise a minimum HRT 
of 2 days is chosen, making the average HRT 4 days at 
average flow. 

Zone 3 should be provided the same design consider¬ 
ations as zone 1, since it functions in the same manner. 
Depending on the performance of zone 2, it may also pro¬ 
vide denitrification in addition to flocculation and sedimen¬ 
tation. Therefore, it should have approximately the same 
HRT as zone 1. 

The minimum HRT at Q max is therefore 2 + 2 + 2 = 6 
days, and the average 12 days with the assumptions cho¬ 
sen in this example. This would then require (using equa¬ 
tion 4-4)an overall wetland area of: 

A w = (t)(Q)/(h)(e) 

= (12 d)(18,920 m 3 /d) / (0.8 m)(0.80)(10,000 m 2 /ha) 
= 35 ha 


ings to meet these effluent concentrations are 45 kg BOD 
/ha-d and 30 kg TSS /ha-d. Using the same influent condi¬ 
tions in the first example, the steps of preliminary sizing 
are the same. 

Step 1 - Apply areal loading to determine the FWS system’s 
critical sizing conditions using equation 4-12. 

BOD: at Q ava , A w = 

at Q™.. A » = 

TSS: at Q a>a , A a = 

al Q™,- A „ - 

The limiting condition is again the TSS loading at Q ave , 
where 44 hectares are required. This is a larger require¬ 
ment than in the previous example, as would be expected 
since more stringent effluent requirements are being met. 

Step 2 - Determine the theoretical HRT (t) required for the 
entire 3-zone FWS system and each specific zone 
using equation 4-4, assuming an overall average depth 
(h) of 0.8 m and an overall porosity (e) of 0.8. 

f Q _ (44 ha)(l 0,000 m 2 / ha)(0.8 m)(0.8) 

ave ’ ’ 19,920 m 3 /d 

= 14.9 days 


(18.920) (50)(1000)/(45)(106) 
21 ha 

(37.840) (40)(1000)/(45)(106) 
34 ha 

(18.920) (70)(1000)/(30)(106) 
44 ha 

(37.840) (30)(1000)/(30)(106) 
38 ha 


Applying the normal additional area for buffers and set¬ 
backs of 1.25 to 1.4, the total area required for the FWS 
facility is 45 ha (135 acres). 

Step 3. - Configuration 


forQ ma, t = 7.4 days 

Returning to individual zones and assuming an equal 
minimum HRT in each, equation 4-4 is used in dimension¬ 
ing at maximum flow conditions: 


Given the high TSS of the influent stream and the po¬ 
tential for short circuiting, the system should be designed 
with two parallel treatment trains of a minimum of three 
cells in each. The first cells in each train should normally 
each get 50% of the flow and may have a deeper (1.0 m) 
inlet area directly adjacent to the inlet structure to handle 
any discrete solids settling which might occur at this loca¬ 
tion. This design option would add approximately one day 
to the overall HRT, and would only be chosen in situations 
where pretreatment is likely to allow escape of readily settle- 
able particulates. Multiple cells allow for redistribution of 
the primary cell effluent in the subsequent cell which re¬ 
duces short-circuiting. Flexible intercell piping will facili¬ 
tate maintenance without a major reduction in the neces¬ 
sary HRT to produce satisfactory effluent quality. Aspect 
ratios of the cells should be greater than 3:1 and adapted 
to the site contours and restrictions. Additional treatment 
will likely be required after the FWS system to meet fecal 
coliform and dissolved oxygen permit requirements. 

Design Example 2 - BOD and TSS < 20mg/L Effluent 
Requirements 

To meet this effluent quality an open-water zone will be 
required in the FWS. From Section 4.4.3 the critical load¬ 


(2.5 d)(37,840 m 3 / d) 

2 (1.0)(1.2 m)( 10,000 m 2 / ha) 

= 7.9 ha 

Therefore, the area for zones 1 and 3 are: 

A 3 =A, = (44 - 7.5 )/2 
= 18 ha 

The overall FWS system area, including buffers, would 
be about 58 hectares (145 acres). 

Step 3 - Configuration 

Again the use of parallel trains is encouraged for all the 
same reasons as noted in the previous example. Parallel 
trains of 3 cells in each are recommended which allow any 
single cell in a train to be removed from service with trans¬ 
fer of its influent to the same zone cell. 

By using an aspect ratio in the range of 3 to 5:1 and j 
complete-cell-width inlets and outlets the intercellular trans- 1 
fers should be simplified. 


76 






Design Example 3 - Estimating from examples 1 and 2. 

For design example 1, if the influent to the FWS system 
was facultative lagoon effluent, it would be reasonable to 
expect the characterization in Table 4-6,with cognizence 
that the impact of climate and season on the performance 
of lagoons and the characteristics of municipal wastewa¬ 
ters vary by orders of magnitude. However, the numbers 
in the table are within the normally expected ranges. To 
test the areal loading information presented earlier in this 
chapter, the loading factors for each chemical constituent 
can be computed using equation 4-12. These yield aTKN 
loading of 10.8 kg/ha-d for example 1 and 8.6 kg/ha-d for 
example 2. Similarly, the TP loadings are 2.7 kg/ha-d and 
2.1 kg/ha-d for examples 1 and 2, respectively. 

Comparing these loadings to Figure 4-5, it is impossible 
to accurately predict effluent quality for example 1, but it 
appears that little removal could be expected had the origi¬ 
nal fully vegetated approach been taken. Mechanistically, 
the primary mechanisms which are available for TKN re¬ 
moval in zone 1 are flocculation and sedimentation of or¬ 
ganic N. Since there are only 4 mg/L of organic N the real¬ 
istic maximum expectation would be a removal of all but 1 
mg/L, while the NH 4 -N may be mostly assimilated by the 
plants during the active growing rate computed for this 
example, but most would be returned to the water column 
during senescence. Therefore, the effluent TKN could av¬ 
erage anywhere between about 15 and 20 mg/L,depending 
on the season, and an overall removal of about 2 to 3 mg/ 
L. With the final open-water design of this example, it is 
not possible to predict removal without more data from 
open-zoned systems. As in example 2, the three-zone FWS 
is likely to produce an effluent TKN of greater than 4 mg/L 
since all three such systems on the figure were loaded at 
a lower rate. Actual removal would depend upon nitrifica¬ 
tion accomplished in zone 2, which would be a function of 
temperature, HRT and dissolved oxygen in that zone. Since 
the TKN loading rate is about 4 to 5 times the highest one 
in the figure for open water systems, a conservative ap¬ 
proach might be that one-fourth of the nitrogen might be 
nitrified in the open zone and denitrified in zone 3. This 
would yield an additional 4 mg/L to the 3 assumed for ex¬ 
ample 1. This would yield a removal of 7 mg/L and an ef¬ 
fluent TN of about 13 mg/L which could vary from about 10 
to 16 mg/L during the year depending on plant condition 
and temperature. Conversely, the systems shown in the 
figure may have had excess capacity in zone 2 to fully 
nitrify all the ammonium-nitrogen and the same effluent 
concentration for the lightly loaded open-water systems 
could also be attained. By having an open-water zone 
where nitrification can occur, the inherent denitrification ca- 


Table 4-6. Lagoon Influent and Effluent Quality Assumptions. 


Parameter 

Raw Wastewater 

Lagoon Effluent 

TKN(mg/L) 

40 

20 

NH 4 -N(mg/L) 

10 

16 

TP(mg/L) 

7 

5 

FC(#/100ml) 

106 

104 


pability of the subsequent fully-vegetated zone creates a 
potent opportunity for nitrogen control. It is also feasible 
in the open-water zone to enhance NH3 volatilization, but 
this mechanism is less likely to be significant owing to the 
limited size of zone 2 which may not permit increased pH 
which would enhance volatilization. Such estimates are 
extremely tenuous until more data is generated on higher 
loadings to these open water FWS systems, and in the 
interim the designer would be wise to perform pilot studies 
where nitrogen limits are part of effluent permit require¬ 
ments. 

Areal loading data on total phosphorus (TP) in Figure 4- 
6 are inconclusive. The TADB (USEPA, 1999) would sug¬ 
gest that these example loadings could produce an efflu¬ 
ent of 3.0 to 4.5 mg/L. At the loadings indicated in these 
examples, the data of Gearheart (1993) would allow for an 
overall annual average removal of approximately one mg/ 
L. This would provide a similar effluent for both examples 
of about 4 mg/L. The dominant removal mechanisms in 
both examples are flocculation and sedimentation of or¬ 
ganic phosphorus, but plant uptake and release will cause 
the effluent to vary from background levels in the growth- 
phase to levels at or above the influent concentration dur¬ 
ing the senescent-phase. This discussion does not include 
the startup-phase where TP removal will occur for several 
months until the soil’s phosphorus adsorption capacity is 
reduced to an equilibrium level by satisfaction of the soil’s 
calcium, aluminum and iron adsorption sites and comple¬ 
tion of the initial growth phase of the plants. 

Fecal coliform (FC) removal is limited by the natural back¬ 
ground which is depicted in Table 4-4. Figure 4-10 shows 
that FC removal is based on enhanced sedimentation and 
flocculation in the fully-vegetated zone of an FWS. There¬ 
fore, approximately one log (90%) of removal can be safely 
estimated in that zone. With an open-water zone, the FWS 
can take advantage of the natural solar disinfection which 
is described in the international lagoon literature (Mara, 

1975). This additional kill of FC is limited by the HRT in the 
open-water zone and is a temperature-dependent func¬ 
tion with first-order kinetics. Time limitation and single-cell 
hydraulics will likely limit additional kill to about one log. 
Since in zone 3 FC removal would be by sedimentation, 
less than one additional log of removal could be expected. 

Based on the prior analyses the total removal for ex¬ 
amples 1 and 2 would be 2+ logs of kill, with an expected 
effluent FC count of <100/100ml. A fully vegetated system 
which has no open zone would likely remove somewhere 
between 1 and 2 logs to produce an effluent with several 
hundred FC/100ml. Both would experience a natural varia¬ 
tion about those means as discussed earlier in the chap¬ 
ter. One major reason for periodic increases will be wildlife 
attraction to open water zones. However, with the require¬ 
ment that the outlet be located at the terminus of the sub¬ 
sequent fully-vegetated zone 3, the impacts of wildlife 
should be minimized. However, some spikes of fecal 
coliform may still be evident. 

As noted earlier, the impact of the example designs on 
metals and toxic organics will vary also. Most metals will 


77 











likely be removed with the TSS by physical means, and 
effluent metals will probably be similar in ratio to the two 
TSS effluent concentrations. The average pond effluent 
had a TSS of 70 mg/I and the two FWS designs should 
yield 30mg/L and 20mg/L, respectively. Therefore, design 
examples 1 and 2 should remove about 70% of the heavy 
metals. Since each metal reacts differently this type of 
analysis has little meaning. Typically, nickel, boron, sele¬ 
nium and arsenic are more resistent to removal by sedi¬ 
mentation than most of the other commonly measured 
metals. A similar discusion can be provided with regard to 
semi-volatile toxic organic compounds. Both classes of 
pollutants may be associated with certain effluent particle- 
size fractions, which will cause them to follow the removal 
patterns discussed earlier in concert with Table 4-3. 

4.6 Design Issues 

This subsection describes issues that are important in 
the design and layout of a FWS constructed wetland. 
These design issues are separate from wetland area de¬ 
terminations already described. However, it is important 
for the engineer/designer to understand that design issues 
and wetland area determinations are both important. The 
design issues outlined here are intended to maximize the 
treatment potential of FWS constructed wetlands and may 
impact the wetland area determined from the wetland siz¬ 
ing steps, which should be considered the starting point 
for a FWS constructed wetland design. Many of the de¬ 
sign issues outlined below are based on experience with 
FWS constructed wetland systems currently in operation. 

4.6.1 Wetland Layout 

4.6.1.1 Site Topography 

In many cases, the topography of the site will dictate the 
general shape and configuration of the FWS constructed 
wetland. On sloping sites, for example, constructing the 
long dimension of the wetland parallel to the existing ground 
contours helps minimize grading requirements. With proper 
design, sloped sites can reduce pumping costs by taking 
advantage of the existing hydraulic gradients. 

4.6.1.2 Aspect (length to width) Ratio 

The aspect ratio (AR) or length to width ratio (L/W), of a 
FWS wetland system is defined as the average length di¬ 
vided by the average width, and can be expressed as: 



where: 

L = average length of wetland system, and 

W = average width of wetland system. 

FWS constructed wetlands have been designed with ARs 
from less than 1:1 to over 90:1. Generally, FWS constructed 
wetlands are designed and built with an AR greater than 
1:1. It has been suggested that wetlands with higher ARs 
help to minimize short circuiting, and force the wetland to 


more closely conform to plug flow hydraulics (Gearheart, 
1996: Dombeck, 1998). However, results of dye studies 
on existing FWS constructed wetlands have shown that 
many wetlands deviate from ideal plug flow hydraulics in¬ 
dependent of the AR. For wetland systems with very high 
length to width ratios, careful consideration needs to be 
given to headloss and hydraulic gradient considerations 
to avoid overflows of confining dikes near the influent end. 
Use of equation 4-6 and the material in section 4.3.2.2 will 
permit the designer forced to use high AR cells to evaluate 
each assumption and make corrections as necessary. 
When conducting a hydraulic grade line analysis to deter¬ 
mine if the backwater is at an acceptable elevation near 
the inlet, the outlet level is normally assumed to be at the 
midpoint. 

4.6.1.3 Wetland configuration 

The shape of a FWS constructed wetland can be highly 
variable depending on site topography, land configuration, 
and surrounding land use activities. FWS constructed wet¬ 
lands have been configured in a number of shapes, in¬ 
cluding rectangles, polygons, ovals, kidney shapes, and 
crescent shapes. There is no data that supports one FWS 
constructed wetland shape as being superior in terms of 
constituent removal and effluent quality, over another 
shape. However, any wetland shape needs to be designed 
and configured following the general guidelines of this re¬ 
port. Design concerns such as hydraulic retention time, 
short-circuiting, headloss, inlet/outlet structures, and inter¬ 
nal and surface configurations can significantly impact 
wetland effluent quality. 

4.6.1.4 Multiple cells 

It has been shown in both the design of oxidation ponds 
and FWS constructed wetlands, that a number of cells in 
series can consistently produce a higher quality effluent. 
This is based upon the hydrodynamic characteristics that 
constituent mass is gathered at the outlet end of one cell, 
and redistributed to the inlet of the next cell. This process 
also minimizes the short circuiting effect of any one unit, 
and maximizes the contact area in the subsequent cell. 
For treatment and water quality purposes, it is recom¬ 
mended that a FWS constructed wetland should consist 
of a minimum of three cells in series. Open water zones 
have also been used to redistribute flows, but their value 
in this regard has been overshadowed by their other at¬ 
tributes. 

Large wetland cells can have internal berms running 
parallel to the flow direction, effectively creating smaller 
parallel cells with better hydraulic properties. Multiple cells 
with appropriate piping between them offer greater opera¬ 
tional flexibility. In the event that a wetland cell needs to be 
taken off line for maintenance reasons, the remaining cell 
or cells can remain operational. This is made even more 
important if cells are sized to coincide with zoning. Com¬ 
pletely vegetated and completely open cells are easier to 
maintain and are more flexible when sequencing or inde¬ 
pendent cell HRT adjustments or maintenance is required. 


78 





4.6.2 Internal Wetland Components 

4.6.2.1 Open water/Vegetation ratio 

The location of emergent vegetation, the type and den¬ 
sity of this vegetation, and the climate as it relates to plant 
senescence are important factors in the design of a FWS 
constructed wetland. Providing adequate open water ar¬ 
eas is an important, but often overlooked, component in 
the design and implementation of FWS constructed wet¬ 
lands (Gearheart, 1986; Hammer, 1996; Hamilton, 1994, 
Stefan et a!., 1995). Open water is defined as a wetland 
surface which is not populated by emergent vegetation 
communities, but may contain submergent aquatic plants 
as well as unconsolidated groupings of floating aquatic 
plants. Historically, many FWS constructed wetlands were 
designed and built as fully vegetated basins with no desig¬ 
nated open water areas. Many of these systems proved 
problematic with very low or no water column dissolved 
oxygen, that resulted in odor production and vector prob¬ 
lems. 

Natural wetlands generally contain a mix of open water 
and emergent vegetation areas. The open water areas 
provide many functions such as oxygenation of the water 
column from atmospheric reaeration, submerged macro¬ 
phytes, and algal photosynthesis. They also permit preda¬ 
tion of mosquito larvae by fish and other animals and pro¬ 
vide habitat and feeding areas for waterfowl. Open water 
areas in FWS constructed wetlands will not only provide 
the same functions as for natural wetlands, but will also 
provide the opportunity for increased soluble BOD reduc¬ 
tion and nitrification of wastewater because of the increase 
in oxygen levels. It is recommended that a FWS constructed 
wetland not be fully vegetated, but should include some 
open water areas. Open water areas in a FWS constructed 
wetland will result in a more complex, dynamic, and self- 
sustaining wetland ecosystem, that better mimics a natu¬ 
ral wetland. Open water wetlands have lower background 
BOD than fully vegetated wetlands (see Table 4-4), which 
reflects their improved treatment potential. 

The ratio of open water to emergent vegetation depends 
on land availability, costs, and the function and goals of 
the FWS constructed wetland system. Generally speak¬ 
ing zones 1 and 3 should be 100% vegetated with the zone 
2 surface having > 50% to 100% open water. If denitrifica¬ 
tion is required, the 3rd zone which is 100% vegetated will 
accomplish it.. The open-water zones with an HRT in ex¬ 
cess of 3 days may invite algal blooms. As long as this 
zone is followed by a fully vegetated zone with an HRT of 
2 or more days, this should not represent a problem be¬ 
yond increased biomass management requirements. 

The most effective method used for creating open water 
areas in a single cell is to excavate a zone that is deep 
enough to prevent emergent vegetation colonization and 
migration. Some have periodically raised water levels to a 
depth that limits emergent vegetation growth, but this is 
operationally demanding and may have negative treatment 
impacts. The type of dominant macrophyte (i.e., emer¬ 


gent or submergent) can be controlled by controlling the 
operating depth. Water column depths greater than ap¬ 
proximately 1.25 to 1.5 meters planted with submergents 
such as Potomogeton spp., will not rapidly be encroached 
upon by emergent macrophytes like bulrushes reeds, and 
cattails. If the water column depth is between 0.5 to 1.0 
meters and planted with emergent vegetation, such as 
bulrush and cattails, they will prevail over submergents 
and most other emergents by filling in the surface area 
through rhizome and tuber propagation. The seasonal 
change in water levels (hydroperiod) is also a determinant 
in establishing various aquatic macrophyte communities. 

Due to the lack of shading, significant blooms of algae 
can occur in large open water areas, which can have nega¬ 
tive effects on effluent quality. To help minimize the poten¬ 
tial for algal growth, open water areas should be designed 
for less than 2 to 3 days hydraulic retention time. In gen¬ 
eral, the growth cycle of algae is approximately 7 days, so 
providing open water areas with less than 2-3 days reten¬ 
tion time will help minimize algal growth in the open-water 
zone of the wetland. Sufficient standing crops of 
submergent macrophytes may also limit algal regrowth in 
these zones. Conversely, excessive algal growth may im¬ 
pair the performance of the submergent macrophytes by 
limiting the solar energy which reaches them. 

Guidelines for designing a FWS constructed wetland in 
terms of vegetated covering are as follows: Begin with an 
emergent vegetation zone covering the volume used in 
the first 2 days of retention time at maximum monthly flow 
(Q max ) to provide for influent solids flocculation and sepa¬ 
ration.. The emergent zone should be followed by an open 
water zone covering days 3 and 4 in the retention time 
sequence at Q max . The open water zone should be designed 
to facilitate production of dissolved oxygen to meet CBOD 
and NBOD demands. The final 2 days of hydraulic reten¬ 
tion volume at Q max should be an emergent wetland to 
reduce any solids (algae, bacteria, etc.) generated in the 
open water and to supply carbon (decomposing plant ma¬ 
terial) and anoxic conditions for denitrification. It is also 
recommended that this final stand of emergent vegetation 
be as close as possible to the outlet of a FWS constructed 
wetland. This provides a final level of protection just be¬ 
fore the effluent leaves the wetland to minimize the impact 
of wildlife on effluent quality. This recommendation may 
heighten the maintenance requirements for the outlet de¬ 
vice, but it will result in less variability in the effluent qual¬ 
ity. 

4.6.2.2 Inlet Settling Zone 

Depending on the pretreatment process, a substantial 
portion of the incoming settleable and suspended solids 
may be removed by discrete settling in the inlet region of a 
FWS constructed wetland. For example, if the FWS sys¬ 
tem is to follow an existing treatment facility which is prone 
to produce high concentrations of settleable TSS, an inlet 
settling zone should be used. If the pretreatment facility 
does a good job of solids capture, but has a high concen¬ 
tration of soluble constituents, an inlet settling zone is of 


79 








no value. Given that FWS systems are generally preceded 
by lagoon systems which seasonally produce large TSS 
concentrations, primarily due to algal mass, the need for 
an inlet settling zone would be marginal since algal solids 
will require flocculation and sunlight restriction before be¬ 
coming settleable. 

If an inlet settling zone should be desired, it should be 
constructed across the entire width of the wetland inlet. A 
recommended guideline is to design a settling zone which 
provides approximately 1 day hydraulic retention time at 
the average wastewater flow rate, as most settleable and 
suspended solids are removed within this time period. The 
settling zone should be deep enough to provide adequate 
accumulation and storage of settled solids, but shallow 
enough to allow the growth of emergent vegetation, such 
as bulrush and cattails. Recommended depth is approxi¬ 
mately 1 meter. 

Most accumulated organic solids will slowly decay and 
reduce in volume. This decay is one of the two major 
sources of internal loading and background constituents 
in the effluent. However, at some time in the future the 
remaining accumulated solids will need to be removed from 
the settling zone. 

4.6.2.3 Inlet/Outlet Structures 

Placement and type of inlet and outlet control structures 
are critical in FWS constructed wetlands to ensure treat¬ 
ment effectiveness and reliability. To effectively minimize 
short-circuiting in a FWS constructed wetland, two goals 
concerning cell inlet/outlet structures are critical: (1) uni¬ 
form distribution of inflow across the entire width of the 
wetland inlet, and (2) uniform collection of effluent across 
the total wetland outlet width. Both of these should mini¬ 
mize localized velocities, thus reducing potential 
resuspension of settled solids. It is important that any out¬ 
let structure be designed so that the wetland can be com¬ 
pletely drained, if required. Some of the common types of 
wetland inlet/outlet systems in use today, and general 
guidelines regarding their design are further discussed in 
Chapter 6. 

Depending on the type of wastewater influent, the inlet 
structure discharge point could be located below or above 
the wetland water surface. Perforated pipe inlet/outlet struc¬ 
tures can be difficult to operate and maintain when they 
are fully submerged. All inlet distribution systems should 
be accessible for cleaning and inspection by using 
cleanouts. 

Outlet structures represent an operational control fea¬ 
ture that directly affect wetland effluent quality. It is impor¬ 
tant that outlet structures facilitate a wide range of operat¬ 
ing depths. By adjusting the outlet structure, both the wa¬ 
ter depth and hydraulic retention time can be increased or 
decreased. This and the need to accommodate cell drain¬ 
age usually results in locating the outlet manifold at the 
bottom of the outlet zone. The differences in water quality 
between water depths can also be highly variable. An outlet 


structure design which allows for maximum flexibility of 
collection depths may be desirable, but may not always 
be compatible with collection devices that minimize short- 
circuiting. With this type of design, the outlet structure can 
be adjusted to draw wetland effluent from the water depth 
with the best water quality. This alternative design usually 
involves multiple drop boxes with openings at different 
depths. In most cases however the uniform collector set 
at the bottom is favored owing to its inherent advantages 
in terms of improved effluent quality and facilitation of cell 
drainage. 

Two types of inlet/outlet structures are commonly used 
in FWS constructed wetlands. For small or narrow (high 
AR) wetlands, perforated PVC pipe can be used for both 
inlet and outlet structures. The length of pipe should be 
approximately equal to the wetland width, with uniform 
perforations (orifices) drilled along the pipe. The size of 
the pipes, and size and spacing of the orifices will depend 
on the wastewater flow rate and the hydraulics of the inlet/ 
outlet structures. It is important that the orifices be large 
enough to minimize clogging with solids. Perforated pipes 
can be connected to a manifold system by a flexible tee 
joint, which allows the pipes to be adjusted up or down. In 
some cases wetland designers with this type of inlet/outlet 
structure will cover the perforated pipes with gravel to pro¬ 
vide more uniform distribution or collection of flows. This 
type of inlet/outlet structure requires periodic inspection, 
some operation and maintenance to maintain equal flow 
through the pipe, and access at the end to clean clogged 
orifices. 

For larger wetland systems, multiple weirs or drop boxes 
are generally used for inlet and outlet structures. Weirs or 
drop boxes are generally constructed of concrete, but 
smaller PVC boxes are also available. These structures 
should be located no further apart than every 15 m (center 
to center) across the wetland inlet width, with a preferred 
spacing of 5 to 10 m. The same spacing requirements apply 
for the outlet weirs or drop boxes. Depending on the source 
of the wastewater influent, the inlet weirs or drop boxes 
can be connected by a common manifold pipe. Whatever 
the configuration, it is important that the manifold pipes 
and weirs be hydraulically analyzed to attain reasonably 
uniform distribution. Simple weir or drop box type inlet struc¬ 
tures are relatively easy to operate and maintain. 

Weir overflow rates have not been considered in the 
design of most wetlands. Weir loading rates of existing 
wetlands are significantly higher than those required in 
most biological solids removal processes (i.e., 120 to 190 
m 3 /m.d ) (WEF, 1998). Excess weir rates can cause high 
water velocities near the outlet which could entrain solids 
which would otherwise be removed from the effluent. 
Therefore, weir loading rates should be designed to meet 
the above range for best performance until more quantita¬ 
tive data are generated. 

4.6.2.4 Baffles 

Baffles are internal structures installed either perpen¬ 
dicular or parallel to the direction of flow. Baffles can be 


80 


ids, 






effective in reducing short-circuiting, for mixing waters of 
different depths, and for improved flocculation performance. 
Properly designed and placed open water zones can also 
act as baffles by allowing mixing and redistribution of waste- 
water before it enters into the next wetland vegetated zone. 
The use of baffles depends on cell configurations, aspect 
ratios, treatment goals, and permit compliance. In general, 
except for special circumstances unforseen in typical mu¬ 
nicipal wastewater treatment application the use of such 
structures is not recommended. However, their use in cor¬ 
recting problems which are due to hydraulic flow difficul¬ 
ties (short circuiting, dead zones, etc.) make them useful 
to the operator-owner. 

4.6.2.5 Recirculation 

Recirculation is the process of introducing treated efflu¬ 
ent back to the inlet or to some other internal location of 
the wetland. Recycling effluent can decrease influent con¬ 
stituent concentrations and increase dissolved oxygen 
concentrations near the inlet. The increased dissolved 
oxygen concentrations can help reduce inlet odors, lower 
BOD, and enhance nitrification potential in open-water 
zones. If recirculation is to be considered, the effects of 
recirculation on the wetland water balance and wetland 
hydraulics need to be analyzed. In general, the ability to 
recycle, like the ability to drain each cell, could be consid¬ 
ered part of the need to have flexible piping , multiple cells, 
and multiple trains of cells. The value of recirculation has 
not been shown to date to be a major factor in improving 
FWS performance. 

4.6.2. 6 Flow Measuring Devices 

Many existing wetland systems do not have accurate 
flow measuring devices. Even if accurate estimates of in¬ 
flows and/or outflows to the treatment plant are known, 
internal flow distribution to individual wetland cells is not 
known or measured. Without accurate flow measurements 
to individual wetland cells, it is impossible to determine 
internal flow rates, average velocities, and hydraulic re¬ 
tention times for each cell, thus making system perfor¬ 
mance adjustments difficult. It is recommended that some 
type of flow measuring device be either installed in or avail¬ 
able to be installed in each cell of a FWS constructed wet¬ 
land. This includes separate flow measuring devices on 
each inlet for multiple wetland cell configurations. Some 
examples of flow measuring devices include simple 90 g V- 
notch or rectangular weirs, and more sophisticated Parshall 
flumes for larger systems. Depending on the size and lay¬ 
out of the wetland, cell inlet/outlet structures should be 
designed to be compatible with available flow measuring 
devices. 

4.6.3 Pretreatment Requirements 

Examples of treatment that should precede FWS con¬ 
structed wetlands include all types of stabilization ponds 
and primary sedimentation systems. The use of wetlands 
to polish secondary effluent to less than 10 mg/I BOD and 
TSS has been documented, but is not covered in detail 
here. The reader is directed to USEPA (1999) for guid¬ 


ance in these applications. The effluent entering a FWS 
constructed wetland should be free from floatable and large 
settleable solids, and excessive levels of oil and grease. 
Also important to a FWS constructed wetland is the in¬ 
coming metal concentrations. While a FWS constructed 
wetland does remove and immobilize many heavy metals 
along with the TSS, excessive influent concentrations could 
result in residuals which are unacceptable for subsequent 
land application. A source reduction program and/or an 
industrial waste pretreatment ordinance are required if 
excessive metals concentrations are present in the raw 
wastewater. 

4.7 Construction/Civil Engineering Issues 

Specific construction/civil engineering design issues that 
should be considered early in the planning and design 
phase of a FWS constructed wetland project include site 
topography and soils, berm construction, impermeable liner 
materials, wetland vegetation substrate, and internal drain¬ 
age. Many of these issues should be considered during 
the site selection process, as they may become difficult or 
costly to correct later in the actual design and construction 
phases of the project. The construction/civil engineering 
requirements for a FWS constructed wetland are similar 
to other earthen water quality management systems such 
as oxidation ponds, and are discussed in Chapter 6 and in 
USEPA (1983) and Middlebrooks, et al (1982). 

4.7.1 Site Topography and Soils 

In general, level land with clay soils affords the optimal 
physical setting for a FWS constructed wetland. Potential 
wetland sites with other physical conditions can be used, 
but may require more substantial engineering, earthwork, 
construction requirements, and liners. In order to overcome 
site limitations, the cost of a FWS constructed wetland will 
also increase proportionally as the wetland site further 
deviates from optimal site conditions. 

FWS constructed wetlands can be built on sites with a 
wide range of topographic relief. Construction costs are 
lower for flat sites since sloped sites require more grading 
and berm construction. Site topography will generally dic¬ 
tate the basic shape and configuration of the FWS con¬ 
structed wetland. 

The principal soil considerations in siting and implement¬ 
ing a FWS constructed wetland are the infiltration capacity 
of the soils and their suitability as berm material and wet¬ 
land vegetation substrate. In most cases FWS constructed 
wetlands are required to meet stringent infiltration restric¬ 
tions depending on the state regulations for groundwater 
protection. An exception are wetland systems designed to 
incorporate infiltration as part of the treatment and dis¬ 
charge process. In these cases, the underlying soil must 
have infiltration rates compatible with the design discharge 
rates. If native site soils are not suitable, separate infiltra¬ 
tion trenches can be added to increase the infiltration sur¬ 
face area. In some cases, it will be necessary to import 
berm and/or bottom materials or use synthetic liners (see 
Chapter 6) to prevent infiltration. 


81 








Interior berms containing FWS wetland cells should be 
built with up to 3:1 side slopes as the soil characteristics 
allow. A minimum freeboard of 0.6 m above the peak flow 
operating depth in the wetland is required. For wetlands 
that will receive exceptionally high peak inflows, additional 
freeboard may be required to ensure that berm overtop¬ 
ping does not occur. Additional freeboard may also be de¬ 
signed to accommodate long-term solids and peat buildup 
during the operation of the wetland, and to allow appropri¬ 
ate water depths to be maintained as sludge builds up in 
the initial cells over time. 

All FWS-cell external berms should have a minimum top 
width of 3 m, which provides an adequate road width for 
most standard service vehicles. In some cases, internal 
berms can have smaller top widths, as routine operation 
and maintenance can be carried out by small motorized 
vehicles, such as ATVs. Road surfaces should be an all 
weather type, preferably gravel, which also minimizes di¬ 
rect runoff into the wetland. 

Berm integrity is critical to the long term operational ef¬ 
fectiveness of FWS constructed wetlands. Common berm 
failure causes include burrowing by mammals, such as 
beaver nutria and muskrat, and holes from root penetra¬ 
tion by trees and other vegetation growing on or near the 
berms. Several design features can eliminate and/or mini¬ 
mize these problems. A thin impermeable wall, or internal 
layer of gravel, can be installed during construction, which 
will minimize mammal burrowing and/or root penetration. 
Planting the berm using vegetation with a shallow root sys¬ 
tem can also be effective. Unlike oxidation ponds, berm 
erosion in fully vegetated zones and/or cells from wave 
action is generally not a concern due to the dampening 
effect of the wetland vegetation. However, in larger cells 
with open zones it could be an issue, and stabilization pond 
texts should be consulted for solutions (Middlebrooks, et 
al, 1982)(USEPA, 1983). 

In the design and site selection process, an important 
consideration is the amount of additional area required for 
berms. In general, the higher the aspect ratio for a FWS 
constructed wetland, the more area that will be required 
for the berms and for the entire wetland system. This in¬ 
crease in required total wetland area to accommodate 
berms is more pronounced for smaller wetlands than for 
larger wetlands. A factor of 1.2 to 1.4 times the cell area is 
usually employed to determine the total site area for the 
FWS system. 

4.7.2 Impermeable Liner Materials 

A concern with FWS constructed wetlands is the poten¬ 
tial loss of water from infiltration and contamination of 
groundwater below the wetland site. While there are some 
wetland applications where infiltration is desirable, the 
majority of the applications require some type of barrier to 
prevent groundwater contamination. Under ideal condi¬ 
tions, the wetland site will consist of natural soils with low 
permeability that restrict infiltration. However, many wet¬ 
lands have been constructed on sites where soils have 


high permeability. In these cases, some type of liner or 
barrier will likely be required to minimize infiltration. Liner 
requirements can also add significantly to the construction 
cost of a FWS constructed wetland. 

Existing natural soils with permeability less then approxi¬ 
mately 10~ 6 cm/s are generally adequate as an infiltration 
barrier. For site soils with higher permeabilities, some type 
of liner material will likely be required. Some examples of 
wetland liner materials include imported clay fill, bentonite 
soil layers, chemical treatment of existing soils, asphalt, 
and synthetic membrane liners such as PVC or HDPE. In 
some instances, it will be possible to compact the existing 
site soils to acceptable permeability. Due to their ability to 
be placed in shaped wetland cells, clay liners are gener¬ 
ally a more sustainable component of the wetland than 
synthetic membrane liners. Whatever liner material is cho¬ 
sen, an important consideration is to provide adequate soil 
cover and depth that protects the liner from incidental dam¬ 
age and root penetration from the wetland vegetation (see 
Chapter 6). 

4.7.3 Soil Substrates for Plants 

Aquatic macrophytes generally reproduce asexually by 
tuber runners. Soils with high humic and sand components 
are easier for the tubers and runners to migrate through, 
and plant colonization and growth is more rapid. The soil 
substrate for wetland vegetation should be agronomic in 
nature (e.g. loam), well loosened, and at least 150 mm (6 
inches) deep. Depending on the liner material, deeper soil 
substrates may be required to protect the liner. If this type 
of soil layer exists at the site, it should be saved. After the 
wetland basin, berms and other earthen structures are 
constructed, and the liner is installed (if required), the origi¬ 
nal soil substrate can be placed back into the excavated 
region. To meet soil specifications, it may be necessary to 
amend the saved soils with other materials. 

While soils such as loam and silt are good for plant 
growth, they can allow large vegetation mats to float when 
large water level fluctuations occur in the wetland. Float¬ 
ing vegetation mats can significantly alter the treatment 
capabilities of FWS constructed wetlands by allowing 
wastewater to flow between the floating mats and substrate, 
not in contact with any vegetation treatment media. To cir¬ 
cumvent this potential problem, denser soil substrates such 
as a sandy loam, or a loam gravel mix can be used. This 
will be more important in FWS constructed wetlands where 
large water depth fluctuations will be part of the operation 
and maintenance procedure. 

4.7.4 Internal Drainage and Flexible 
Piping 

In the event a FWS constructed wetland needs to be 
drained, the wetland bottom should have a slope of 1% or 
less. Drainage may be required for maintenance reasons 
such as liner repair, sludge removal, vegetation manage¬ 
ment, and berm repair. Deeper channels may be employed 
to allow for drainage and/or continued use when serial cells 
are taken out of service. Channels can also be used to 


82 




connect deeper open water areas where these are part of 
a larger cell, rather than separate cells. In general the more 
complete the intercellular piping, the greater the opera¬ 
tional flexibility is for the entire system. 

4.8 Summary of Design Recommendations 

A summary of the design recommendations for FWS 
wetland treatment systems is presented in Table 4-7. As 
more quality-assured data become available allowable 
pollutant areal loadings will likely be revised. 


Table 4-7. Recommended Design Criteria for FWS Constructed 
Wetlands 


Parameter 

Design Criteria 

Effluent Quality 

BOD < 20 or 30 mg/L 

TSS < 20 or 30 mg/L 

Pretreatment 

Oxidation Ponds (lagoons) 

Design Flows 

Q max (maximum monthly flow) and 
Q™ (average flow) 

Maximum BOD Loading 
(to entire system) to Meet: 

20 mg/L: 45 kg/ha-d 

30 mg/L: 60 kg/ha-d 

Maximum TSS Loading 
(to entire system) to Meet: 

20 mg/L: 30 kg/ha-d 

30 mg/L: 50 kg/ha-d 

Water Depth 

0.6 - 0.9 m Fully vegetated zones 
1.2-1,5m Open-water zones 

1.0m Inlet settling zone 

(optional) 

Minimum HRT (at Qmax) 
in Zone 1 (and 3) 

2 days fully vegetated zone 

Maximum HRT (at Qave) 
in Zone 2 

2 - 3 days open-water zone 
(climate dependent) 

Minimum Number of Cells 

3 in each train 

Minimum Number of Trains 

2 (unless very small) 

Basin Geometry (Aspect Ratio) 

Optimum 3:1 to 5:1, but subject to 
site limitations 

AR > 10:1 may need to calculate 
backwater curves 

Inlet Settling Zone Use 

Where pretreatment fails to retain 
settleable particulates 

Inlet 

Outlet 

Uniform distribution across cell inlet 
zone 

Uniform collection across cell outlet 
zone 

Outlet Weir Loading 

<200 m3/m-d 

Vegetation 

Emergent - 

Typha or Scirpus (native species 
preferred) 

Submerged - 

Potamogeton, Elodea, etc (see 
chapter 2). 


Table 4-7. Continued 


Parameter 

Design Criteria 

Design Porosities 

0.65 for dense emergents in fully 
vegetated zones 

0.75 for less dense stand of 


emergents in same zones 

1.0 for open-water zones 

Cell Hydraulics 

Each cell should be completely 
drainable 

Flexible intercell piping to allow for 
required maintenance 

Independent, single-function cells 
could maximize treatment 


4.9 References 

Balmer, P. and B.Vik.. 1978. “Domestic Wastewater Treat¬ 
ment with Oxidation Ponds in Combination with Chemi¬ 
cal Precipitation,“ Prog. Water Tech, Vol 10, No. 5-6, 
pp867-880. 

Carre, J., M. P. Loigre, and M. Leages 1990. “Sludge Re¬ 
moval from Some Wastewater Stabilization Ponds.” 
Water Science Technology, Vol 22, No 3-4, pp 247- 
252. 

Cole, S. 1988. "The Emergence of Treatment Wetlands". 
ES&T. Vol 3, No. 5, pp. 218-223. 

Crites, R.W., and G. Tchobanoglous. 1998. “Small and De¬ 
centralized Wastewater Management Systems, WCB 
- McGraw-Hill, NY. 

Dombeck, G. 1998. Sacramento Regional Wastewater 
Treatment Plant Demonstration Wetland Project. 1997 
Annual Report, Nolte and Associates, Sacramento, Ca. 

Frankenbach, R.l and J.S Meyer. 1999. Nitrogen Removal 
in A Surface Flow Wetland Wastewater Treatment Wet¬ 
lands, 1999, Wetlands, Volume la, No. 2, June 1999 
pp.403-412. 

Gearheart, R.A., and B. Finney. 1999. “The Use of Free 
Surface Constructed Wetlands as An Alternative Pro¬ 
cess Treatment Train to Meet Unrestricted Water Rec¬ 
lamation Standards”, Wat. Sci. Tech. Vol. 40, No. 4-5, 
pp. 375-382. 

Gearheart, R.A., B. A. Finney, M. Lang, and J. Anderson. 
1998. “A Comparison of System Planning, Design and 
Sizing Methodologies for Free Water Surface Con¬ 
structed Wetlands”. 6th International Conference on 
Wetland Systems for Water Pollution Control. 

Gearheart, R.A. and B. A. Finney. 1996. Criteria for De¬ 
sign of Free Surface Constructed wetlands Based 
Upon a Coupled Ecological and Water Quality Model. 
Presented at the Fifth International Conference on 
Wetland Systems for Water Pollution Control, Vienna, 
Austria. 


83 








Gearheart, R. A. 1995. Watersheds - Wetlands - Wastewa¬ 
ter Management. In Natural and Constructed Wetlands 
for Wastewater Treatment. Ramadori, R., L. Cingolani, 
and L. Cameroni, eds., Perugia, Italy, pp 19-37. 

Gearheart, R. A. 1993. “Phosphorus Removal in Constructed 
Wetlands”. Presented at the 66th WEF Conference and 
Exposition, Anaheim, Ca. 

Gearheart, R.A. 1992. Use of Constructed Wetlands to Treat 
Domestic Wastewater, City of Areata, California”, Wat. 
Sci. Tech., Vol. 26, No. 7-8, pp. 1625-1637. 

Gearheart, R.A., F. Klopp, and G. Allen. 1989. Constructed 
Free Surface Wetlands to Treat and Receive Wastewa¬ 
ter Pilot Project to Full Scale, In D.A. Hammer (ed.) Con¬ 
structed Wetlands for Wastewater Treatment, pp. 121- 
137, Lewis Publisher, Inc., Chelsea, Ml 

Gearheart, R.A., B. A. Finney, S. Wilbur, J. Williams, and D. 
Hall. 1984. ‘The Use of Wetland Treatment Processes 
in Water Reuse”, Future of Water Reuse, Volume 2, Pro¬ 
ceedings of Symposium III Water Reuse, AWWA Re¬ 
search Foundation, pp. 617-638. 

Gearheart, R.A., S. Wilbur, J. Williams, D. Hull, B. A. Finney, 
and S. Sundberg. 1983. City of Areata Marsh Pilot 
Project: effluent quality results-system design and man¬ 
agement. Final report. Project No. C-06-2270, State 
Water Resources Control Board, Sacramento, CA. 

Gregg, J., and A. Horne. 1993. "Short-term Distribution and 
Fate of Trace Metals in a Constructed Wetland Receiv¬ 
ing Treated Municipal Wastewater", Environmental En¬ 
gineering and Health Sciences Laboratory Report No. 
93-4. University of California, Berkeley, CA. 

Hammer, D, 1992,"Creating Freshwater Wetlands", Lewis 
Publishers, Chelsea, Ml 

Hannah, S.A, B.M. Austern, A.E. Eralp, and R.H. Wise, 1986. 
Journal WPCF, Vol 55, No 1, pp 27-34. 

Hovorka, R.B. 1961. An Asymmetric Residence-time Distri¬ 
bution Model for Flow Systems, Dissertation, Case In¬ 
stitute of Technology. 

Kadlec. R.H. 2000. The Inadequecy of First-Order Treatment 
Wetland Models. Ecological Engineering, vol. 15, pp 105- 
109; 

Kadlec, R.H. and R.L. Knight. 1996. Treatment Wetlands. 
Boca Raton, FL: Lewis-CRC Press. 

Kadlecik, L. 1996. Organic Content of Wetland Soils, Areata 
Enhancement Marsh, Special Project, ERE Department 
Wetland Workshop. 

Levine, A. D., G. Tchobanoglous and T. Asano, 1991. Size 
Distributions of Particulate Contaminants in Wastewa¬ 
ter and Their Impact on Treatability. Water Research, 
Vol. 25, No 8, pp. 911-922. 


Linsley, R.K. Jr., M.A. Kohler, and J.L.H. Paulhus, 1982. 
Hydrology for Engineers, 3rd Ed., McGraw-Hill, NY. 

Mara, D.D. 1975 Proposed Design for Oxidation Ponds in 
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300. 

Marais, C. V. R., and V. A. Shaw. 1961. A Rational Theory for 
the Design of Sewage Stabilization Ponds in Central and 
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Middlebrooks, E. J., C. E. Middlebrooks, T. H. Reynolds, G.Z. 
Watters, S.C. Reed, and D.B.George. 1982. Wastewa¬ 
ter Stabilization Lagoon Design, Performance and Up¬ 
grading, MacMillen, New York, NY. 

Mitsch, W.J. and J.G. Gosselink.1993. Wetlands. Van 
Nostrand Reinhold, NY. 

NADB (North American Treatment Wetland Database). 1993. 
Electronic database created by R. Knight, R. Ruble, R. 
Kadlec, and S. Reed for the U.S. Environmental Protec¬ 
tion Agency. Cincinnati, OH. 

Odegaard, H. 1987. Particle Separation in Wastewater Treat¬ 
ment. In Proceedings EWPCA 7th European Sewage 
and Refuse Symposium, pp. 351-400. 

Reckhow, K., and S. S. Qian. 1994. Modeling Phosphorus 
Trapping in Wetlands Using General Models. Water 
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fornia Constructed Wastewater Treatment Wetland, Eco¬ 
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801-811. 

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84 




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City of San Diego Pilot-Scale Aquatic Wastewa¬ 
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1625-1655. 

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85 










Chapter 5 

Vegetated Submerged Bed Systems 


5.1 Introduction 

The pollutant removal performance of vegetated sub¬ 
merged bed (VSB) systems depends on many factors in¬ 
cluding influent wastewater quality, hydraulic and pollut¬ 
ant loading, climate, and the physical characteristics of the 
system. The main advantage of a VSB system over a free 
water surface (FWS) wetland system is the isolation of the 
wastewater from vectors, animals and humans. Concerns 
with mosquitoes and pathogen transmission are greatly 
reduced with a VSB system. Properly designed and oper¬ 
ated VSB systems may not need to be fenced off or other¬ 
wise isolated from people and animals. Comparing con¬ 
ventional VSB systems to FWS systems of the same size, 
VSB systems typically cost more to construct, primarily 
because of the cost of media (Reed et al., 1985). Because 
of costs, it is likely that the use of the conventional VSB 
systems covered in this manual will be limited to individual 
homes, small communities, and small commercial opera¬ 
tions where mosquito control is important and isolation fenc¬ 
ing would not be practical or desirable. 

A conventional VSB system is described in Chapter 2 
and depicted in Figure 2-3. The typical components in¬ 
clude (1) inlet piping, (2) a clay or synthetic membrane 
lined basin, (3) loose media filling the basin, (4) wetland 
vegetation planted in the media, and (5) outlet piping with 
a water level control system. The vast majority of VSB sys¬ 
tems have used continuous and saturated horizontal flow, 
but several systems in Europe have used vertical flow. 

Alternative VSB systems are defined here as VSBs that 
have been modified to improve their treatment performance 
(George et al., 2000, Young et al., 2000, Behrends et al., 
1996). Typical modifications involve some type of cyclic 
filling and draining of the system to improve the oxygen 
input into the media. The potential improvement in perfor¬ 
mance with alternative VSB systems is offset to some de¬ 
gree by a more complex and expensive operating system. 
It is too early to predict whether alternative VSB designs 
will prove to be more cost effective or practical than con¬ 
ventional VSB systems, although they appear to provide 
significantly better removal of certain pollutants. 

This chapter will discuss VSB systems that treat (1) septic 
tank and primary sedimentation effluents, (2) pond efflu¬ 
ents, and (3) secondary and non-algal pond effluents. The 


most common VSB systems in the U.S. treat septic tank 
and pond effluents for BOD and TSS removal. In Europe, 
VSB systems are most often used to treat septic tank ef¬ 
fluents, although they have also been used extensively in 
the U.K. for polishing activated sludge and RBC effluents, 
and for treating combined sewer bypass flows (Cooper, 
1990, Green and Upton, 1994). 

This chapter provides a summary of the theoretical and 
practical considerations in the design of conventional VSB 
systems. VSB systems, like other natural treatment sys¬ 
tems, are less understood than highly-engineered waste 
treatment systems because they (1) have more variable, 
complex, and less controllable flow patterns, (2) have re¬ 
action rates and sites within the system that vary with time 
and location, and (3) are subject to the inconsistencies of 
climate and growth patterns. This complexity makes the 
development and use of design equations based on ideal¬ 
ized reactor and reaction kinetic theory difficult, if not im¬ 
practical and unrealistic. Furthermore, because pollutant 
removal performance can be quite variable, designs must 
be conservative if a guaranteed effluent quality is required. 

5.2 Theoretical Considerations 

5.2.1 Potential Value of Wetland Plants in 
VSB Systems 

In several recent studies that have compared the pollut¬ 
ant removal performance of planted and unplanted VSB 
systems, it has been found that plants do not have a major 
impact on performance (Young et al, 2000, George et al., 
2000, Liehr et al., 2000). There is however significant cost 
and time associated with the establishment and mainte¬ 
nance of the wetland plants in a VSB system. Neverthe¬ 
less, planted systems have a significant aesthetic advan¬ 
tage over unplanted systems and may be of value as wet¬ 
land habitat in some cases. Unfortunately, the aesthetic 
value of plants and the value of VSB wetland systems as 
wetland habitat are difficult to quantify, and no mitigation 
credit is given by the USEPA for the habitat value they 
provide. In the following sections the potential value of 
wetland plants in VSB systems is discussed in more de¬ 
tail. 

5.2.1.1 Type of Wetland Plants 

Several studies have attempted to determine if pollutant 
removal performance differs with various types of wetland 


86 




plants (Gersberg et al., 1986, Young et al., 2000). Although 
some researchers have claimed a relationship, these 
claims have not been substantiated by others (Gersberg 
et al., 1986). 

It is not clear if it is desirable to maintain a single plant 
species, or a prescribed collection of plant species, for any 
treatment purpose. Single plant (monoculture) systems are 
more susceptible to catastrophic plant death due to pre¬ 
dation or disease (George et al., 2000). It is generally as¬ 
sumed that multiple plant and native plant systems are 
less susceptible to catastrophic plant death, although no 
studies have confirmed this assumption. Plant invasion and 
plant dominance further complicate the issue; in several 
cases researchers have found that, with time and without 
operator intervention, one of the planted species or an in¬ 
vader species has become the dominant species in all or 
part of the system (Young et al., 2000, Liehr et al., 2000). 
This occurs less frequently and more slowly in VSB sys¬ 
tems than in FWS systems. 

The impact of wetland plants on pollutant removal per¬ 
formance appears to be minimal based on current knowl¬ 
edge, so the selection of plants species should be based 
on aesthetics, impacts on operation, and long-term plant 
health and viability in a given geographical area. Local 
wetland plants experts should be consulted when making 
the selection. 

5.2.1.2 Plant Mediated Gas Transfer 

Wetland plants can facilitate gas transfer both into and 
out of the wastewater of a VSB system. The focus of most 
studies has been oxygen transfer into the wastewater. 
However, methane and other dissolved gases in the waste- 
water can be transferred out of the wastewater by wetland 
plants. The mechanisms of plant-mediated gas transfer 
are described in detail in Chapter 3. The potential amount 
of oxygen transferred by plant roots into the wastewater 
depends on many factors including dissolved oxygen con¬ 
centration in the wastewater, root depth in the wastewater, 
air and leaf temperatures, and plant growth status (rapid 
growth vs. senescence). Most studies to determine the 
rates of plant-mediated oxygen transfer have been per¬ 
formed in laboratory microcosms or mesocosms under 
controlled conditions.(George et al., 2000, Liehr, et al., 
2000) It is not clear if these results are transferable to full- 
scale systems. 

Based on a review of the literature, the likely rate of oxy¬ 
gen transfer is between zero and 3.0 g-0 2 /m 2 -d (0 - 0.6 
lbs/1000 ft 2 -d). While this maximum value is within the BOD 
loading range of lightly loaded VSB systems, (3 g/m 2 -d = 
30 kgBOD/ha-d = 27 lb BOD/ac-d), there is very little evi¬ 
dence to support the assumption that plants add signifi¬ 
cant amounts of oxygen to VSB systems. Typical values 
of dissolved oxygen in VSB systems are very low (<1 .Omg/ 
L), but because of the difficulty in obtaining an accurate in- 
situ oxygen reading, the actual values are probably even 
lower. In VSB systems where oxidation-reduction poten¬ 
tial (ORP) has been measured, values were typically quite 
negative, indicating strong reducing conditions. 


Unplanted systems have been found to perform as well 
as planted systems in both BOD and ammonia nitrogen 
removal (George et al., 2000, Liehr et al., 2000, Young, et 
al., 2000). Furthermore, investigations of root depth and 
flow pathways have found that the roots do not fully pen¬ 
etrate to the bottom of the media and there is substantially 
more flow under the root zone than through it (Young et 
al., 2000, George et al., 2000, Bavor et al. 1989, Fisher, 
1990, DeShon et al., 1995, Sanford et al., 1995a & 1995b, 
Sanford, 1999, Rash and Liehr, 1999, Breen and Chick, 
1995, Bowmer, 1987). The oxygen supply from the roots 
is also likely to be unreliable due to yearly plant senes¬ 
cence, plant die-off due to disease and pests, and vari¬ 
able plant coverage from year to year. Considering all of 
these factors, it is recommended that designers assume 
wetland plants provide no significant amounts of oxygen 
to a VSB system. 

Plants will also affect the other potential source of oxy¬ 
gen to VSBs — the direct oxygen transfer from the atmo¬ 
sphere to the wastewater. Researchers at TVA have esti¬ 
mated oxygen transfer from the atmosphere to be between 
0.50 and 1.0 g-0 2 /m 2 -d (0.1-0.2 lbs/1000 ft 2 -d) (Behrends 
et al., 1993). Decomposing plant matter on top of the me¬ 
dia would likely cause even lower rates of oxygen trans¬ 
port into the wastewater because the plant matter acts as 
a diffusion barrier and, ultimately, an oxygen demand. 

5.2.1.3 Nutrient and Metals Removal by 
Wetland Plants 

Wetland plants take up macro-nutrients (such as N and 
P) and micro-nutrients (including metals) through their roots 
during active plant growth. At the beginning of plant se¬ 
nescence most of the nutrients are translocated to the rhi¬ 
zomes and roots. A significant proportion of the nutrients 
may also be exuded from the plant (Gearheart et al., 1999). 
Estimates of net annual nitrogen and phosphorus uptake 
by emergent wetland species vary from 12 to 120 gN/m 2 -y 
and 1.8 to 18 gP/m 2 -y respectively (Reddy and DeBusk, 
1985). Reeds and bulrush are at the lower end of both 
ranges while cattails are at the higher end. These esti¬ 
mates are based on annual growth rates and nutrient con¬ 
centrations of the whole plant, but since in a VSB system 
only the shoots can be harvested, the values should be 
reduced by at least 50%. Plant uptake of metals can also 
be estimated by this method. To maximize nutrient removal 
by the plants in a VSB system, shoot harvesting must be 
done before senescence. Harvesting of wetland plants is 
not recommended during the growing season because the 
warm temperatures may cause plant stress, substantial 
stem death, and significant delay in re-growth in some 
wetland plants (George, et al., 2000). 

The expected maximum removal rates of nitrogen, phos¬ 
phorus and metals by direct plant uptake and harvesting 
are small compared to typical loadings in VSB systems. 
Furthermore, nitrogen, phosphorus and metals removal by 
plant uptake will vary with time. Most of the nutrient up¬ 
take occurs during rapid plant growth in the spring and 
summer, and if the plant is not harvested before senes- 


87 





cence a significant portion of the plant-sequestered nutri¬ 
ents are released back into the water. Therefore, unless 
the nutrient removal standards for a VSB system are also 
variable and synchronous with plant uptake and release, 
the presence of plants may be more harmful than helpful 
in meeting nutrient removal standards. Finally, it is unlikely 
that the nutrients or metals removal obtained by harvest¬ 
ing are worth the considerable time and labor required to 
harvest and reuse or dispose of the biomass. 

5.2.1.4 Plant-Supplied Carbon Sources for 
Denitrification 

Because of the inherent anaerobic conditions associ¬ 
ated with VSB systems, they are good candidates for deni¬ 
trification. The likely limiting factor for denitrification in VSB 
systems is biodegradable organic carbon. The value of 
plant-supplied organic carbon for denitrification in a VSB 
system depends on the wastewater COD to nitrogen ratio 
and the forms of nitrogen in the influent to the system. 
Plant-supplied organic carbon is most important in VSB 
systems treating nitrate-rich influents deficient in biode¬ 
gradable organic carbon such as effluents from nitrifying 
activated sludge plants. The minimum COD to nitrate-ni¬ 
trogen ratio for denitrification is 2.3 g-C0D/g-N0 3 -N. Since 
oxygen is used preferentially over nitrate as the electron 
acceptor by the microbes that carry out denitrification, the 
required C0D/N0 3 -N ratio can be significantly higher if any 
oxygen is present in the system. 

Decomposing wetland plants and plant root exudates 
are potential sources of biodegradable organic carbon for 
denitrification but are also sources of organic nitrogen, 
which is easily converted to ammonia. Plant root exudates 
of organic carbon and nitrogen are the largest at the be¬ 
ginning of senescence. Because of the predominantly 
anaerobic conditions in VSB systems, decomposition of 
plant biomass within the media of a VSB system will likely 
provide more organic carbon (and ammonia) to the waste- 
water than will decomposition of the plant biomass on top 
of the media, which takes place in largely aerobic condi¬ 
tions. Some of the decomposition products of biomass on 
top of the media (including nitrates) are transported into 
the wastewater by precipitation infiltration. 

In one study of a VSB system treating a nitrified second¬ 
ary effluent, nitrate removal improved from 30% to 80% 
when mulched biomass including straw, wetland plants, 
and grass was applied to the top of the media (Gersberg 
et al., 1983). Another study with a VSB system treating a 
nitrified landfill leachate found that nitrate removal was lim¬ 
ited by biodegradable organic carbon (Liehr et al., 2000). 

5.2.1.5 Plant Role in Thermal Insulation 

One potential advantage of a planted over an unplanted 
VSB system is the role of plants in providing thermal insu¬ 
lation to the wastewater during cold weather. Dead plant 
biomass on top of the media helps to limit both convective 
heat losses from the wastewater and infiltration of melted 
snow into the wastewater. Two researchers have devel¬ 
oped methods to estimate the effect of plants in prevent¬ 


ing heat loss from the wastewater of a VSB system (Reed 
et al., 1995, Smith et al., 1997). However, it is not clear 
how important this factor is in pollutant removal perfor¬ 
mance because 1) it has not been shown that planted VSB 
systems perform better than unplanted systems, even in 
winter, and 2) the dead plant material on top of the media 
also acts as a barrier to oxygen transfer and a potential 
source of biodegradable carbon and nutrients to the waste- 
water. 

5.2.1.6 Plant Impact on Hydraulic Conductivity 
(Clogging) and Detention Time 

Several VSB systems have experienced conditions 
called “surfacing” where a portion of the wastewater flows 
on top of the media. Surfacing (1) creates conditions fa¬ 
vorable for odors and mosquito breeding, (2) creates a 
potential health hazard for persons and animals that may 
come into contact with the wastewater, and (3) reduces 
the hydraulic retention time (HRT) and performance of a 
VSB system. Surfacing occurs whenever the hydraulic 
conductivity of the media is not sufficient to transport the 
desired flow within the usable headloss of the media. The 
usable headloss is defined by the difference in the eleva¬ 
tions of the outlet piping and the top of the media. Surfac¬ 
ing can result from a number of factors including (1) poor 
design of the system inlet and outlet piping, (2) an inaccu¬ 
rate estimate of the clean hydraulic conductivity of the 
media, (3) improper construction, and (4) an inaccurate 
estimate of the reduction in hydraulic conductivity, or “clog¬ 
ging”, that will occur due to solids accumulation and/or 
growth of plant roots. Several researchers have found that 
clogging was the most severe within the first 1/4 to 1/3 of 
the system (Young et al., 2000, George et al., 2000, Bavor 
et al. 1989, Fisher, 1990, Sapkota and Bavor, 1994, Tan¬ 
ner and Sukias, 1995, Tanner et al., 1998). The hydraulic 
conductivity was found to be less restricted and fairly uni¬ 
form over the remaining length of the system. 

Based on studies in Europe during the 1980s, some re¬ 
searchers proposed that plant roots significantly increased 
the hydraulic conductivity in VSBs with soil media by open¬ 
ing up preferential pathways for the wastewater flow 
(Kickuth, 1981). Later studies of these systems found that 
a significant portion of flow occurred on top of the soil (Coo¬ 
per et al., 1989). Based on recent studies, the presence of 
plant roots in the gravel media of a VSB system will have a 
negative effect on hydraulic conductivity (George et al., 
2000, Young et al., 2000, DeShon et al., 1995, Sanford et 
al., 1995a and 1995b, Breen and Chick, 1995). Research¬ 
ers at TTU compared the reduction in void volume due to 
root and non-root solids. They estimated that the reduc¬ 
tion in void volume due to root solids (2 - 8%) was much 
larger than the reduction in void volume due to non-root 
solids (0.1 - 0.4%). Even though the overall estimated re¬ 
duction in void volume was small, there was a 98% reduc¬ 
tion in hydraulic conductivity. 

The primary functions of a plant’s roots are to supply 
water and nutrients and to physically anchor or support 
the above-ground portions on the plants. Water and nutri- 


88 



ents will be plentiful at all depths of a VSB, so the plant 
roots will typically penetrate only 15 - 25 cm (6“ -10") as 
needed to anchor the plant. In most VSB systems the plant 
roots do not fully penetrate the entire depth of the media 
and the reduction in hydraulic conductivity in the root zone 
results in the creation of “short-circuiting” under the root 
zone, and more flow through the portion of the media with¬ 
out roots (Bavor et al., 1989, Fisher, 1990, DeShon et al., 
1995, Sanford et al., 1995a and 1995b, Sanford, 1999, 
Rash and Liehr, 1999, Tanner and Sukias, 1995, Breen 
and Chick, 1995). This situation may also lead to the cre¬ 
ation of stagnant zones within the media ("dead volume") 
which results in lower actual HRTs as the water preferen¬ 
tially flows through a smaller volume of the media. The 
decrease in HRT will depend in part on the fraction of the 
depth that is occupied by the roots; that is, deeper beds 
will have more a greater proportion of the media that is not 
impacted by roots. 

From tracer studies, researchers have found significant 
differences between actual and theoretical HRTs in their 
VSB systems and attributed it to dead volume in the upper 
zone of the media where the majority of the roots grow 
(Liehr et al., 2000, Young et al., 2000, Bavor et al., 1989, 
Fisher, 1990, DeShon et al., 1995, Sanford et al., 1995a 
and 1995b, Breen and Chick, 1995). However, the re¬ 
searchers at TTU did not find a significant reduction in HRT 
in three of the cells they studied. (George et al., 2000) 

The researchers at North Carolina State University 
(NCSU) attributed part of the dead volume in their sys¬ 
tems to stratification of the water caused by less dense 
rain water infiltration ponding within the media on top of 
higher density leachate. This phenomenon has also been 
reported by others (DeShon et al., 1995, Sanford et al., 
1995a and 1995b, Sanford,1999, Rash and Liehr, 1999). 
NCSU found that the short-circuiting was greater in an 
unplanted VSB system than in a planted system. While 
rain water ponding may be a problem with some VSB sys¬ 
tems, the effect at NCSU was magnified by the relatively 
large catchment area (due to the shallow side slopes used 
in the system) and the salinity of the leachate. The CU 
researchers performed two sets of tracer studies in the 
three cells of the Minoa system. The first set was performed 
in the clean media of each cell before planting. The sec¬ 
ond set was performed after plant establishment on the 
one cell that was half planted and half unplanted. From 
the first study they concluded that there was short circuit¬ 
ing through the lower media and dead volume resulting in 
the actual HRTs being only 75% of the theoretical values, 
even in the clean media. They attributed these results to 
media compaction during construction and intermixing of 
the upper pea gravel with the lower larger media. From 
the second tracer study they concluded that plants roots, 
which penetrated only half of the media depth, resulted in 
more short circuiting and dead volume than in the unplanted 
media. Tanner & Sukias (1995) also reported more accu¬ 
mulation of solids in the root zone, which further contrib¬ 
uted to preferential flow around the root zone. 


5.2.2 Removal Mechanisms 

5.2.2.1 BOD and TSS 

VSB systems have been used for secondary treatment 
(i.e. 30 mg/L of BOD and TSS) for a variety of wastewa¬ 
ters including: primary and septic tank effluents; pond ef¬ 
fluent; and effluents from activated sludge, RBC, and trick¬ 
ling filter systems that don’t consistently meet secondary 
standards. As discussed in Chapter 3, the primary mecha¬ 
nisms for BOD and TSS removal are flocculation, settling, 
and filtration of suspended and large colloidal particles. 
VSB systems are effective for TSS and BOD because of 
relatively low flow velocities and a high amount of media 
surface area. They typically do better at TSS removal, be¬ 
cause TSS removal is a completely physical mechanism, 
while BOD removal is more complex. Larger biodegrad¬ 
able particles that have been quickly removed by physical 
mechanisms will be degraded over time and be converted 
into particles in the soluble and small colloidal size range. 
As such they become an internal “source” of BOD as they 
degrade and reenter the water. Some material is also in¬ 
corporated into microbial biomass. 

Some material will accumulate in a VSB, but the amount 
of long term solids accumulation is unknown. Tanner and 
Sukias (1995) reported finding less solids accumulation 
than would be expected based on the load in the influent 
wastewater. Researchers at Richmond, Australia (Bavor 
et al., 1989, Fisher, 1990) found that most solids were re¬ 
moved in the initial section of the VSB and that the “solids 
accumulation front” stabilized after a year and did not ad¬ 
vance. These findings support the idea that trapped mate¬ 
rial will degrade over time. VSB systems treating pond 
wastewater are likely to accumulate more solids, and be 
more susceptible to clogging, because TSS in pond waste- 
water is predominantly algae, which are slightly less bio¬ 
degradable and degrade more slowly than typical primary 
or secondary wastewater solids. 

BOD and TSS in the effluent from a VSB are probably 
not materials that have passed through the VSB, but rather 
are converted or internally produced material. As such it is 
likely to be quite different in size or composition from influ¬ 
ent BOD and TSS. For example, the influent TSS in the 
Las Amimas system were predominantly algal cells, but 
there were almost no algal cells in the effluent even thought 
the effluent TSS averaged 30 mg/L (Richard & Synder, 
1994). 

True BOD removal only occurs when the material caus¬ 
ing the BOD is completely converted by anaerobic biologi¬ 
cal processes to gaseous end products. The two most likely 
anaerobic pathways are methane fermentation and sul¬ 
fate reduction. Because methane fermentation is severely 
inhibited at temperatures below 10°C, sulfate reduction 
probably predominates for soluble BOD removal during 
colder months. However, seasonal performance does not 
vary as much as would be expected based on the typical 
temperature dependence of biological reactions. A likely 
explanation, illustrated in Figure 5-1, is that biodegradable 
particles that are physically removed during colder months 


89 







BOD (mg/L) TSS (mg/L) 




Figure 5-1. Seasonal cycle in a VSB 


90 















are degraded more slowly and accumulate (Kadlec and 
Knight, 1996). As the temperature warms up the rate of 
degradation of trapped particles increases, leading to a 
reduction of accumulated solids and a release of BOD. 
This theory would explain why summer BOD removal rates, 
based on influent BOD loading, do not appear to be sig¬ 
nificantly greater than winter removal rates. The need for 
insulation of the surface of VSB systems in northern cli¬ 
mates has been discussed, but the need has not been 
quantified (Jenssen, etal, 1993). 

Alternative VSB systems should achieve higher oxygen 
transfer rates, so BOD removal should improve because 
aerobic biological processes will become more prevalent. 
However, microbial biomass production should also in¬ 
crease, which may lead to increased clogging problems. 
The potential of alternative VSB systems for TSS and BOD 
removal is unclear, but performance at Minoa, NY has been 
very good (Reed and Giarrusso, 1999). 

5.2.2.2. Nitrogen 

Several conventional VSB systems have been designed, 
built and operated to remove ammonia from various waste- 
waters. While partial ammonia removal has been achieved 
in some systems, the removals have been less than pre¬ 
dicted (George et al., 2000, Liehr et al., 2000, Young et al., 
2000). Ammonia can be removed by microbial reactions 
or plant uptake. Because VSB systems are predominantly 
anaerobic, microbial removal via nitrification is very lim¬ 
ited. As discussed in Section 5.2.1.3, plant uptake is also 
very limited. Very lightly loaded systems have achieved 
partial ammonia removal (George et al., 2000,1999; Young 
et al., 2000), but if ammonia removal is required, a sepa¬ 
rate ammonia removal process should be used in conjunc¬ 
tion with a VSB system. 

The predominantly anaerobic condition of VSB systems 
seems well suited for microbial removal of nitrate via deni¬ 
trification, but there are relatively few studies to document 
their use for this specific purpose (Gersberg, et al, 1983; 
Stengel and Schultz-Hock, 1989). Systems treating well 
oxidized secondary effluents or other carbon limited waste- 
waters may have inadequate carbon for denitrification to 
proceed efficiently (Liehr et al., 2000). Systems treating 
wastewaters with more carbon, and that have achieved 
partial nitrification, typically achieve almost complete deni¬ 
trification (George et al., 2000, Young et al., 2000). Crites 
and Tchobanoglous (1998) suggest that significant denitri¬ 
fication of municipal wastewaters can occur in VSB sys¬ 
tems at a detention time of 2 to 4 days, but Stengel and 
Schultz-Hock (1989) demonstrated with methanol addition 
that denitrification was carbon limited. 

Alternative VSB systems should achieve higher oxygen 
transfer rates, so they should be more efficient at ammo¬ 
nia removal via nitrification (George et al., 2000, Reed & 
Giarusso, 1999, Behrends et al., 1996, May et al., 1990) 
and less efficient for nitrate removal via denitrification than 
conventional VSBs. 


5.3 Hydrology 

5.3.1 Evapotransporation and 
Precipitation Impacts 

The avoidance of surfacing is a major design criterion 
and high amounts of precipitation or snowmelt can increase 
the flow in a VSB system. In climates with extended peri¬ 
ods of precipitation or heavy snowmelt, the runoff from the 
total catchment area that drains into the VSB must be es¬ 
timated and included in the design flow. Evapotransporation 
(ET) decreases the hydraulic loading and will not contrib¬ 
ute to surfacing. 

Except in very wet climates, flows from precipitation 
events will probably not adversely affect performance be¬ 
cause VSB systems have a relatively small surface area 
(compared to FWS wetlands) and effluent controls should 
be sufficient to prevent surfacing. Precipitation dilutes pol¬ 
lutants in the system, temporarily raises the water level, 
and decreases the HRT, while ET concentrates pollutants, 
temporarily lowers the water level, and increases the HRT. 
ET rates will vary depending on plant species and density, 
but rates from 1.5 to 2 times the pan evaporation rate have 
been reported in the literature (refs). Except in very wet or 
dry climates, the two results are probably offsetting, re¬ 
ducing the overall impact on water level and effluent val¬ 
ues. Unfortunately, the specific effects of ET and precipi¬ 
tation on VSB performance are not documented because 
good estimates of ET and precipitation are hard to obtain, 
and precise influent and effluent flow measurements are 
seldom available, even in research systems. 

5.3.2 Water Level Estimation 

An important step in the design process is to estimate 
the elevation of the water surface throughout the VSB to 
ensure that surfacing of the wastewater does not occur. 
As in all gravity flow systems the water level in a VSB sys¬ 
tem is controlled by the outlet elevation and the hydraulic 
gradient, or slope, which is the drop in the water level 
(headloss) over the length from the inlet to the outlet. The 
relationship between flow through a porous media and the 
hydraulic gradient is typically described by the general form 
of Darcy’s Law (Eq. 5.1). This form assumes laminar flow 
through media finer than coarse gravel, and many authors 
have modified it for other applications including other me¬ 
dia and turbulent flow. However, use of the general form 
without modification is recommended as sufficient to esti¬ 
mate the water level within a VSB. 


Q = (K)(A)(S) = (K)(W)(DJ(dh/dL) 
or, for a defined length of the VSB, 

(5-1) 

dh = (Q)(L) / (K)(W)(D w ) 

(5-2) 


where Q= flow rate, m 3 /d 

K = hydraulic conductivity, m 3 /m 2 -d, or m/d 
A = cross-sectional area normal to wastewater flow, m 2 
C = (W)(DJ 

where W = width of VSB, m 
D w = water depth, m 


91 






L = length of VSB, m 

dh = head loss (change in water level) due to flow re 
sistance, m 

S = dh/dL = hydraulic gradient, m/m 

The water level at the inlet of a VSB will rise to the level 
required to overcome the head loss in the entire VSB. 
Therefore, the VSB must be designed to prevent surfac¬ 
ing. K for an operating VSB varies with time and location 
within the media and will have a major impact on the head 
loss. K is very difficult to determine because it is influenced 
by factors that cannot be easily accounted for, including 
flow patterns (affected by preferential flow and short cir¬ 
cuiting), and clogging (affected by changes in root growth/ 
death and solids accumulation/degradation). Therefore, a 
value must be assumed for design purposes. Typical val¬ 
ues for various sizes of rock and gravel are shown in Table 
5-1. Several of the references listed in Table 5-1 also noted 
that K was much less in the initial 1/4 to 1/3 of the VBS 
than in the remainder of the bed. Based on the studies 
listed in Table 5-1 and many observed cases of surfacing 
in VSB systems, the following conservative values are rec¬ 
ommend for the long-term operating K values: 

initial 30% of VSB K = 1 % of clean K 

final 70% of VSB K ( = 10% of clean K. 

5.3.3 Hydraulic Retention Time and 
Contaminant Dispersion 

The theoretical HRT in any reactor is defined as the liq¬ 
uid volume of the reactor divided by the flow rate through 


it. The liquid volume in a VSB system is difficult to accu¬ 
rately determine because of the loss of pore volume to 
roots and other accumulated solids, such as recalcitrant 
biomass and chemical precipitates. The lost pore volume 
will vary with both location in the VSB and time, both sea¬ 
sonally and yearly, because of root growth and decay, and 
solids accumulation and degradation. Preferential flow (see 
section 5.2.1.6) as illustrated in Figure 5-2 will also have a 
direct impact on HRT and has not been correlated with 
changes in pore volume. For design purposes the volume 
occupied by roots and other solids is assumed to be insig¬ 
nificant and the theoretical HRT is estimated using the 
average flow (including precipitation and ET for very wet 
or dry climates) through the system, the system dimen¬ 
sions, the operating water level, and the initial (clean) po¬ 
rosity of the media, which is either estimated or experi¬ 
mentally determined. 

The actual HRT has been frequently reported to be 40- 
80% less than the theoretical HRT (based on pore vol¬ 
ume) either due to loss of pore volume, dead volume, or 
preferential flow (Fisher, 1990, Sanford et al., 1995b, 
Bhattarai and Griffin, 1998, Batchelor and Loots, 1997, 
Rash and Liehr, 1999, Tanner and Sukias, 1995, Breen 
and Chick, 1995, Tanner et al., 1998, Bowmer, 1987). A 
rough approximation of the liquid volume can be deter¬ 
mined by measuring the volume of water drained from an 
operating bed, but water held in small pores or adhering to 
biomass will remain in the system. Draining will also not 
be able to account for preferential flow. Tracer studies are 
recommended as a more realistic measure of the HRT in 
a VSB system, using one of a variety of tracers (Young et 
al., 2000, Young et al., 2000, George et al., 2000, Netter 


Table 5-1. Hydraulic Conductivity Values Reported in the Literature. 


Size and type' 
of Media 

“CleanTDirty” 

K (m/d) 

Type of Wastewater 
(Typical TSS, mg/L) 2 

Length of 
Operation 

Notes & References 

5-10 mm gravel 

34,000/12,000 

2° effluent (100) 

2 years 

K = 12,000 is for downstream portion (last 80 m) of VSB 

5-10 mm gravel 

34,000/900 

2° effluent (100) 

2 years 

K = 900 is for inlet zone (first 20 m) of VSB 

Bavor et al (1989), Fisher (1990), Bavor & Schulz 
(1993) 

17 mm creek rock 

100,000/44,000 

nutrient solution (neg) 

4 months 

neg = negligible TSS 

6 mm pea gravel 

21,000/9000 

nutrient solution (neg) 

4 months 

Macmanus et al (1992), DeShon et al (1995) 

30-40 mm coarse gravel 

NR/1000 

2° effluent (30 w/a) 

2 years 

w/a = with algae; pond effluent; gravel bed only- 
no plants 

5-14 mm fine gravel 

NR/12,000 

2° effluent (30 w/a) 

2 years 

coarse gravel is first 6m of bed; fine is last 9 m of 
bed Sapkota & Bavor (1994) 

20-40 mm coarse gravel 

NR/NR 

landfill leachate (neg) 

26 months 

for coarse gravel, headloss was controlled by 
outlet, not K 

5 mm pea gravel 

6200/600 

landfill leachate (neg) 

26 months 

Sanford et al (1995a & 1995b), Sanford (1999), 
Surface et al (1993) 

19 mm rock 

120,000/3000 

septic tank effluent (50) 

7 months 

George et al (2000) 

14 mm fine gravel 

15,000/see note 

aerated pond (60 w/a) 

2 years 

K of combined gravel (fine overlaid coarse) was 

22 mm coarse gravel 

64,000/see note 

aerated pond (60 w/a) 

2 years 

2000 at 50 m from inlet; 27,000 at 300 m from inlet 
Kadlec & Watson (1993), Watson et al (1990) 


'Type as defined in the reference(s) 
2 neg = negligible; w/a = with algae 


92 









Wetland Plants 


Jntlet _YT"} 



Outlet 

'Zone 


Intlet 

Zone 


0.6m 


Not to Scale & Dimensions Are “Typical” 


Figure 5-2. Preferential Flow in a VSB 


and Bischofsberger, 1990, Fisher, 1990, Netter 1994, 
Sanford et al., 1995b, Bhattarai and Griffin, 1998, Bowmer, 
1987). 

Some of the current design equations for VSB systems 
assume plug flow conditions. However, tracer studies per¬ 
formed on VSB systems have found significant amounts 
of dispersion as shown in Figure 5-3 (Sanford et al., 1995b, 
Bhattarai and Griffin, 1998, Liehr et al., 2000, George et 
al., 2000). Based on current data it appears that VSB sys¬ 
tems can not be accurately modeled as either plug flow or 
complete mix reactors. The simplest model that can pro¬ 
vide a reasonable fit to the tracer curves is a series of 
equal volume complete mix reactors. However, while this 
model may mathematically fit the tracer data, it does not 
realistically represent physical flow through porous media. 
Intuitively it would seem that a plug flow reactor with dis¬ 
persion would most closely represent the actual conditions 
in a VSB. This model allows greater flexibility in determin¬ 
ing a fit of the tracer data but typically results in a complex 
mathematical model of pollutant removal. Estimates of the 
dispersion number for VSB systems have ranged from 
0.050 to 0.31 (George et al., 2000, Bhattarai and Griffin, 
1998), with greater numbers for systems with small length- 
width ratios. Dispersion numbers less than 0.025 are in¬ 
dicative of near-plug flow conditions while values above 
0.20 indicate a high degree of dispersion. The modeling of 
flow and dispersion is complicated by the non-uniformity 


of flow and pore volume in space and time as previously 
discussed, and by other factors including precipitation and 
ET. 

At this point in time there appears to be little justification 
for using complex flow models, because of a lack of data 
and the unpredictable and constantly varying conditions 
within a VSB. 

5.4 Basis of Design 
5.4.1 Introduction 

Attempting to fully describe pollutant removal in VSB 
systems is at least as complex as trying to describe VSB 
hydraulics. Many authors have examined several relation¬ 
ships as a model for pollutant removal, including zero and 
first order reactions in both plug flow and complete mix 
reactor models. None of the relationships were found to 
reasonably fit of the all data that are available. Further¬ 
more, data from VSB systems are typified by a wide vari¬ 
ability, as would be expected of dynamic natural systems 
that are influenced by many factors. This variability is evi¬ 
dent in the plots of TSS, BOD, TKN and TP data in this 
section. Data scatter is not reduced by comparing pollut¬ 
ant removal with a variety of factors (e.g. area, volume, 
HRT, percent removals or loading rate), or by normalizing 
the data (C e /C 0 ). Expected trends, such as temperature 
dependence for BOD removal or better removal with lower 

















D) 

E 

c 

o 

fc 

k- 

c 

a> 

o 

c 

o 

o 

E 

3 



Figure 5-3. Lithium Chloride Tracer Studies in a VSB System (George et al., 2000) 


pollutant loading, are often not apparent due to the scatter 
of the data. Therefore, the design approach recommended 
here is to use the maximum pollutant loading rates that 
have been shown to meet discharge standards. This ap¬ 
proach yields a much more conservative design than other 
common design approaches. As additional quality data be¬ 
comes available in the future, it may be possible to extend 
these conservative loading rates with confidence. 

Two types of pollutant loading rates were considered, 
an areal loading rate (ALR), g/m 2 -d, and a volumetric load¬ 
ing rate (VLR), g/m 3 -d. Both ALRs and VLRs have been 
used by researchers to describe VSB performance. ALR 
is calculated by multiplying the influent flow rate (m 3 /d) by 
the influent pollutant concentration (mg/L = g/m 3 ), and di¬ 
viding by the surface area of the VSB system (m 2 ). Be¬ 
cause sedimentation, plant growth and oxygen transfer are 
theoretically dependent on the surface area, ALR may be 
a characteristic parameter for some pollutants. VLR is cal¬ 
culated by multiplying the influent flow rate (m 3 /d) by the 
influent pollutant concentration (mg/L = g/m 3 ), and divid¬ 
ing by the pore volume of the VSB system (m 3 ). Because 
the removal of certain pollutants could be dependent on 
the HRT, the VLR could be a characteristic parameter for 
some pollutants. However, because the actual saturated 
pore volume is seldom known and the HRT may not be 
directly related to the pore volume due to preferential flow, 
the utility of VLR for design purposes is limited. Also, a 
comparison of Figures 5-4 through 5-7, which are typical 
of scatter for all pollutants, shows that data scatter is not 


reduced by the use of VLR. Therefore, the design recom¬ 
mendations in this chapter are based on ALRs. 

Finally, because the type of pre-treatment has a major 
impact on the characteristics of the wastewater being 
treated, the following discussions are organized by the type 
of wastewater being treated: septic tank and primary efflu¬ 
ents, pond effluents, and secondary treatment effluents. 

5.4.2 TSS and BOD Removal for Septic 
Tank and Primary Effluents 

Two recent studies, one conducted by Tennessee Tech¬ 
nological University (TTU) and one conducted by Clarkson 
University (CU) at the Village of Minoa, New York, have 
provided the majority of data used to establish the design 
recommendations for this section (George et al., 2000, 
Young et al., 2000). These two studies were chosen be¬ 
cause their research objectives were to provide design in¬ 
formation, they utilized several VSBs with different mea¬ 
sured loadings, and the data are of good quantity and qual¬ 
ity. Influents in the TTU and CU studies were respectively 
a low strength septic tank effluent and a fairly typical pri¬ 
mary effluent. Each data point in the following figures rep¬ 
resents a quarterly average of biweekly (every 2 weeks) 
sampling for the TTU data, and a quarterly average of at 
least two monthly samples for the CU data. The results 
from one other VSB system treating septic tank effluent 
studied by University of Nebraska - Lincoln (UNL) research¬ 
ers are also included in these figures (Vanier & Dahab, 
1997). After reviewing the literature, no other studies with 


94 










50 


40 


o) 30 
E 


★ ★ 

• TTU1 

A TTU2 

★ TTU3 

▼ UNL 

♦ CU 

■ NADB 

• 








TSS Areal Loading Rate (g/m2-d) 


Figure 5-4. Effluent TSS vs areal loading rate 


50 


40 


30 


O) 

E 

CO 

co 


© 20 

3= 

111 


10 


0 


7^2 


n 


r* 


o 


—i— 

100 


• TTU1 
A TTU2 

★ TTU3 
▼ UNL 

♦ CU 


—i-1-1 

200 300 400 

TSS Volumetric Loading Rate (g/m3-d) 


500 


600 


Figure 5-5. Effluent TSS vs volumetric loading rate 


95 




























c 

d) 

J3 

3 = 

LU 


70 


60 


50 


g 40 
Q 

o 

CD 


• 

TTU1 

A 

TTU2 

★ 

TTU3 

▼ 

UNL 

♦ 

CU 

■ 

NADB 



BOD Areal Loading Rate (g/m2-d) 


Figure 5-6. Effluent BOD vs areal loading rate 



O 

O 

m 

c 

< 1 > 

I3 

LU 



BOD Volumetric Loading Rate (g/m3-d) 


Figure 5-7. Effluent BOD vs volumetric loading rate 


96 














































septic tank or primary wastewater were found to have data 
with similar quality and quantity as these three studies. 
Data from the NADB for VSBs treating primary effluent, 
some of which are of unknown quality, are also shown in 
Figures 5-4 and 5-6. 

TSS removal is quite good; effluent TSS was consis¬ 
tently less than 30 mg/L at TSS ALRs as high as 20 g/m 2 - 
d (Figure 5-4). The two data points in Figure 5-4 that are 
above 30 mg/L are from systems at TTU that were inten¬ 
tionally overloaded to failure, and are not typical. Other 
researchers have reported plugging of the surface of the 
media (as opposed to clogging of the pore volume) when 
excessively high TSS loadings were applied (Tanner & 
Sukias, 1995, Tanner et al., 1998, van Oostrom & Cooper, 
1990). Additional data may extend these limited ALRs when 
it becomes available. However, it should be noted that the 
typical sustained influent TSS concentrations for the data 
plotted in these figures were less than 100 mg/L. It is rec¬ 
ommended that TSS ALR be limited to 20 g/m 2 -d, based 
on the maximum monthly influent TSS. This would corre¬ 
spond to a loading of 2 cm/d for an influent concentration 
of 100 mg/L of TSS, 4 cm/d for an influent concentration of 
50 mg/L of TSS, and so on. 

BOD removal is not as good as TSS removal, so the size 
of a VSB designed to meet secondary treatment standards 
will generally be controlled by the requirements for BOD re¬ 
moval. Effluent BOD values were found to periodically ex¬ 
ceed 30 mg/L at BOD ALRs greater than 6 g/m 2 -d (Figure 5-6). 

It is recommended that BOD ALR be limited to 6 g/m 2 -d, 
based on the maximum monthly influent BOD, to produce 


a maximum effluent BOD of 30 mg/L. Table 5-2 compares 
the size of a VSB designed with this ALR compared to the 
size of VSBs designed using several common approaches. 
As expected the other design approaches result in VSB 
systems significantly smaller than that using the conser¬ 
vative design approach presented here. 

5.4.3 Nutrient Removal for Septic Tank 
and Primary Effluents 

Most of the organic nitrogen in septic tank and primary 
effluents is associated with suspended solids that are easily 
removed in VSB systems. It is generally assumed that the 
organic nitrogen will be converted to ammonia in VSB sys¬ 
tems, but spiked concentrations of urea (a soluble form of 
organic nitrogen) were often not completely converted in 
one study (George et al., 2000). Ammonia removal in VSB 
systems is severely oxygen limited, and it is inversely re¬ 
lated to the ultimate (carbonaceous and nitrogenous) BOD 
loading. Also, the conversion of organic nitrogen into am¬ 
monia via ammonification or hydrolysis masks any attempt 
to relate ammonia removal to other design factors. For this 
reason Total Kjeldahl Nitrogen (TKN) data rather than 
ammonia data are presented in Figure 5-8. The TKN re¬ 
moval performance is generally poor and highly variable. 
Therefore, VSB systems should not be used alone to treat 
pre-settled municipal wastewaters if significant amounts 
of ammonia must be consistently removed. 

Although the data are not presented here, if any nitrate 
is produced in VSB systems treating septic tank and pri¬ 
mary effluents, it is likely that the nitrate will be removed 
by denitrification. 


Table 5-2. Comparison of VSB Area Required for BOD Removal Using Common Design Approaches. 
Design criteria 

Flow (Q) = 400 m 3 /d (105,680 gpd) 

Influent BOD 5 (Ci) = 125 mg/L 
Effluent BOD 5 (Ce) = 30 mg/L 


Design 

Approach 

Rate 

Constant 

Loading 

Constant 

Other Factors 

Required 

Area 
m 2 (ac) 

This Manual 


6 g/m 2 -d 
(54 Ib/ac-d) 


8,330 (2.0) 

European 
(Cooper, 1990) 

K BO o = 0Vd 



5710(1.4) 

Kadlec & 

Knight 

(1996) 


180 m/yr 
(590 ft/yr) 

Background Concentration 2 = 10 mg/L 

1420 (0.4) 

Reed, etal. 

(1995) 

Temperature 

Dependent 2 

K10 = 0.62/d 

K20 = 1.104/d 


Water Depth 1 = 0.4 m (16") 

Media Porosity’ = 0.38 

at 10°C, 6090 (1.5) 
at 20°C, 3400 (0.8) 

TVA (1993) 


5.3 g/m 2 -d 
(48 Ib/ac-d) 

Derived from TVA design 

Assumes septic tank effluent 

9430 (2.3) 


’Values chosen by user; these are not necessarily the values recommended by the design’s author. 
2 Values calculated per instructions of design’s author. 


97 







50 


40 


t A 

A 

A ± • 

• 

•: 


★ 

★ * 

• L '.*! 

»♦ •: 

• 

• 

★ 

★ 

♦ 

▲ 

★ 


O) 

E 


c 

d) 

LU 


30 


20 


10 


4 t 

1U» 4 * 


♦ 

★ 


0 


2 


T- 

4 


—i— 

10 


• TTU1 
A TTU2 

★ TTU3 
T UNL 

♦ CU 


—t— 

12 


0 


6 8 
TKN Areal Loading Rate (g/m2-d) 


14 


Figure 5-8. Effluent TKN vs areal loading rate 


Although phosphorus is partially removed in VSB sys¬ 
tems treating septic tank and primary effluents, VSBs are 
not very effective for long-term phosphorus removal (Fig¬ 
ure 5-9). It should be noted that the phosphorus data shown 
in Figure 5-9 are from VSB systems that are relatively new, 
when it can be assumed that the phosphorus precipitation 
and adsorption capacity of the media would be at its great¬ 
est. Because plant uptake of phosphorus is quite small 
compared to typical loadings (Reed, et al, 1995; Crites & 
Tchobanoglous, 1998), the phosphorus removal capacity 
will decrease with time. Estimates of realistic long-term 
phosphorus removal by plant harvesting is limited to about 
0.055 g/m 2 -d (0.5 Ib/ac-d) (Crites & Tchobanoglous, 1998). 
VSB systems should not be expected to remove phospho¬ 
rus on a long-term basis. 

5.4.4 TSS, BOD and Nutrient Removal for 
Pond Effluents 

There is much less quality data comparable to the TTU 
and CU studies for VSB systems treating pond effluents. 
Data from a study conducted at three experimental VSB 
systems at Las Animas, Colorado by Colorado State Uni¬ 
versity (Richard & Synder, 1994) were used to support 
design recommendations for VSB systems treating pond 
effluents (Table 5-3). The pollutant removal performance 
for the Las Animas VSBs treating oxidized pond effluent 
was not as good as the performance of the TTU and CU 
systems. NABD data for VSBs treating pond effluent (Fig¬ 
ure 5-10), which are not as reliable as the Las Animas 


data, show similar performance. Several of the NABD sys¬ 
tems have experienced surfacing caused by clogging of 
the media surface (as opposed to clogging of the pore vol¬ 
ume) by algae. 

For Las Animas, the average TSS ALR was 6.2 g/m 2 -d 
(55 Ib/ac-d) and produced an overall average effluent TSS 
of 35 mg/L. The average BOD ALR was 2.0 g/m 2 -d (18 lb/ 
ac-d) and produced an overall average effluent BOD of 25 
mg/L. There was essentially no nitrogen or phosphorus 
removal on average in the three VSBs. The poor overall 
percent removal of BOD of 35% might be related to the 
relatively high concentrations of algal cells in the influent 
during several months of each year. The measured BOD 
of pond effluent typically does not account for the true BOD 
of algal cells because algal cells degrade more slowly than 
other organic matter. 

A VSB system in Mesquite, Nevada has been used since 
1992 in parallel with overland flow and oxidation ditch sys¬ 
tems to treat an aerated pond effluent. One year of monthly 
data for the Mesquite system is summarized in Chapter 8. 
Over the one-year sampling period the effluent BOD aver¬ 
aged 29 mg/L when loaded at an average BOD ALR of 2.5 
g/m 2 -d (22 Ib/ac-d). Better BOD and TSS removals than at 
Las Animas and Mesquite are reported in a 1993 EPA re¬ 
port for several VSB systems treating pond algal effluents. 
However, the sparse amount of data of unproven quality 
represented by the average values given in the report is 


98 















3.5 



c 

a> 


3 = 

LU 


3 

2.5 

2 

1.5 

1 

0.5 

0 


♦ 

4 ~*-♦- 


♦ ★ 


- ,-*-- 

A • # ♦ 

. # * 

_•. •!. . 1 

•• • * 

• TTU1 
▲ TTU2 

★ TTU3 

♦ CU 




•. * 

i 



l-1-1-1-r 


0 0.2 0.4 0.6 0.8 1 1.2 


TP Areal Loading Rate (g/m2-d) 


Figure 5-9. Effluent TP vs areal loading rate 


inadequate to use with confidence. Sapkota and Bavor 
(1994) report similar TSS removal, but do not report BOD 
removal. 

The upgrading of pond effluent with rock filters is similar 
to the use of VSBs after ponds. However, because of vari¬ 
able results from rock filters, their use is generally cau¬ 
tioned due to a lack of reliable design information (Reed, 
et al, 1995). Performance of rock filters is also plagued by 
H S generation and high effluent ammonia. Illinois requires 
effluent aeration and recommends disinfection before dis¬ 
charge for pond rock filter systems. Because of the limited 
data and uncertainty about similar rock filter systems, VSB 
systems are not recommended for treating pond effluents 
if the system must consistently meet a 30/30 standard. 

5.4.5 BOD, TSS and Nutrient Removal for 
Secondary and Non-algal Pond 
Effluents 

There are very few quality-assured data available from 
VSB systems in the U.S. treating secondary effluents. The 
1993 U.S. EPA report included data collected over a three 
month period from three systems. Additional data from one 
of these systems, Mandeville, LA, is included in Chapter 
8. The Mandeville VSB treats an aerated lagoon effluent 
which has little or no algae. The average influent BOD and 
TSS were 40 and 16 mg/L, respectively. The average ef¬ 
fluent BOD and TSS were 5 and 3 mg/L, respectively at a 
BOD ALR of 7.9 g/m2-d (70 Ib/ac-d). 


Representatives from Severn Trent Water, Ltd., have 
reported on the performance of VSB ("reed bed") systems 
treating activated sludge and RBC effluents in small treat¬ 
ment plants (less than 2000 people) in the U.K. (Green 
and Upton, 1994). The goal for these VSB systems is to 
provide additional treatment of secondary effluents so that 
they consistently meet discharge limitations, which can vary 
from 30/20 to 15/10 TSS/BOD. Essentially these systems 
serve as aesthetic and sometimes economical substitutes 
for tertiary filters for small treatment plants. In some cases 
in the U.K. they have been used to treat storm water by¬ 
pass flows at secondary treatment plants. While the Severn 
Trent systems typically remove some nitrogen and phos¬ 
phorus, they are not capable of meeting typical discharge 
standards for nutrients in the U.S. The primary design ba¬ 
sis used by Severn Trent is a hydraulic surface loading 
rate of 0.20 m 3 /m 2 -d (5 gpd/sq ft) for the average daily flow 
(Green and Upton, 1994). This value is derived from the 
design recommendation of the European task group on 
VSB systems (Cooper, 1990). For systems with an aver¬ 
age influent BOD < 40 mg/L, this results in average areal 
BOD loading of less than 8.0 g/m 2 -d (71 Ib/ac-d). Typical 
systems are 0.6 m (24 in) deep, 0.4 m wide per m 3 /d (5 ft 
per 1000 gpd) of flow, and 12.5 m (41 ft) long. 

Based on the success of the Mandeville and Severn Trent 
systems, it appears that VSB systems can be effectively 
used to help small secondary systems consistently meet 
secondary effluent standards. The recommended approach 


99 






















Table 5-3. Data from Las Animas, CO VSB Treating Pond Effluent. 


Time 

Period 1 

Inf. TSS 
mg/L 

Eff. TSS 
mg/L 

Inf. BOD 
mg/L 

Eff. BOD 
mg/L 

Inf. TKN 
mg/L 

Eff. TKN 
mg/L 

Inf. TP 
mg/L 

Eff. TP 
mg/L 

Cell 1 

Winter 91 

89.7 

43.0 

26.7 

37.5 

ND 

ND 

1.57 

1.9 

Srping 92 

146.0 

28.7 

34.0 

22.7 

7.2 

11.4 

1 

1.67 

Summer 92 

178.0 

41.7 

49.7 

29.0 

6.1 

8.7 

1.17 

1.68 

Fall 92 

223.0 

34.3 

54.0 

30.0 

7.3 

10.7 

1.47 

1.9 

Winter 92 

50.0 

34.3 

33.7 

32.7 

14.1 

14.0 

2.25 

2.3 

Spring 93 

66.0 

24.0 

41.0 

32.0 

16.1 

16.3 

2.55 

2.6 

Summer 93 

95.3 

33.3 

30.0 

9.3 

3.9 

5.3 

0.67 

0.78 

Fall 93 

127.0 

38.0 

40.3 

16.0 

4.0 

3.4 

0.65 

0.6 

Average 

121.9 

34.7 

38.7 

26.2 

8.4 

10.0 

1.42 

1.68 

Cell 2 

Winter 91 

89.7 

51.0 

26.7 

37.0 

ND 

ND 

1.57 

2.57 

Srping 92 

146.0 

34.0 

34.0 

35.0 

7.2 

14.1 

1 

2.37 

Summer 92 

178.0 

43.3 

49.7 

29.3 

6.1 

9.0 

1.17 

1.72 

Fall 92 

223.0 

26.0 

54.0 

40.0 

7.3 

8.8 

1.47 

1.78 

Winter 92 

50.0 

33.0 

33.7 

29.7 

14.1 

14.2 

2.25 

2.28 

Spring 93 

66.0 

26.7 

41.0 

35.3 

16.1 

15.2 

2.55 

2.68 

Summer 93 

95.3 

27.0 

30.0 

15.0 

3.9 

5.5 

0.67 

0.82 

Fall 93 

127.0 

33.3 

40.3 

21.3 

4.0 

4.8 

0.65 

0.65 

Average 

121.9 

34.3 

38.7 

30.3 

8.4 

10.2 

1.42 

1.86 

Cell 3 

Winter 91 

89.7 

46.0 

26.7 

11.3 

ND 

ND 

1.57 

1.47 

Srping 92 

146.0 

47.0 

34.0 

22.0 

7.2 

5.4 

1 

1.43 

Summer 92 

178.0 

33.3 

49.7 

20.3 

6.1 

9.6 

1.17 

1.62 

Fall 92 

223.0 

41.3 

54.0 

27.0 

7.3 

8.1 

1.47 

1.72 

Winter 92 

50.0 

34.3 

33.7 

25.7 

14.1 

13.0 

2.25 

2.2 

Spring 93 

66.0 

23.7 

41.0 

29.7 

16.1 

13.9 

2.55 

2.53 

Summer 93 

95.3 

29.3 

30.0 

8.3 

3.9 

4.7 

0.67 

0.82 

Fall 93 

127.0 

34.3 

40.3 

8.0 

4.0 

5.6 

0.65 

0.55 

Average 

121.9 

36.2 

38.7 

19.0 

8.4 

8.6 

1.42 

1.54 


'Each of the values in the table is the average of three monthly samples. 



BOD or TSS Areal Loading Rate (g/m2-d) 
NADB Systems Treating Pond Effluent 


Figure 5-10. NADB VSBs Treating Pond Effluent 


100 


















is to limit the BOD ALR to a maximum monthly value of 8 
g/m2-d (71 Ib/ac-d). However, VSB systems are not rec¬ 
ommended as a remedy for inadequately operated acti¬ 
vated sludge systems. Process upsets in poorly operated 
activated sludge systems can quickly fill a VSB system 
with mixed liquor solids, resulting in surface flow due to 
clogging of the media. 

5. 4.6 Metals Removal for AII Types of 
Wastewater 

Metals are removed in a VSB by two primary mecha¬ 
nisms. First, because many metals (e.g. Zn, Cr, Pb, Cd, 
Fe, Al) are associated with particles (Heukelekian & Balmat, 
1959; SWEP, 1985), the high efficiency of particulate sepa¬ 
ration in a VSB should remove these metals accordingly. 
Second, sulfide precipitation occurs due to the reduction 
of sulfates to sulfides in the absence of nitrate, and ren¬ 
ders some metals insoluble, resulting in significant remov¬ 
als, as described in Reed, etal (1995) for Cu, Cd, and Zn. 
As long as the system ORP remains low, which is likely 
given the anaerobic nature of VSBs, it is unlikely that met¬ 
als precipitated in the sulfide form will re-enter the water 
column (Bounds, et al, 1998; Reed, et al, 1995). Some 
metals such as Ni and Cd are more mobile and less likely 
to be removed, but they are not normally present in toxic 
quantities in municipal wastewater. 

There is relatively little data on metals removal by VSB 
systems and no known information from long-term stud¬ 
ies. Gersberg et al. (1984) found significant removal of Cu, 
Zn and Cd, and determined that plant uptake was respon¬ 
sible for only 1 % of the Cu and Zn removal. In a study with 
a VSB system treating a landfill leachate, researchers found 
only a small increase in the Pb, Cd, and Cu levels on root 
surfaces, and no increase in any of the metals measured 
in any plant tissue compared to plants from a control sys¬ 
tem (Peverly et al., 1995). They concluded that the in¬ 
creased metal concentrations on the root surface was due 
to metal precipitation and adsorption. Metal removal by 
plant uptake should not be counted on in any VSB system 
over the long term. 

5.4.7 Pathogen Remo val for AII Types of 
Wastewater 

While pathogens will be partially removed in a VSB sys¬ 
tem, a disinfection step after the VSB will normally be re¬ 
quired to meet discharge limits. Researchers in Nebraska 
found a three log reduction in fecal coliforms from 10 6 to 
10 3 /100mL in a VSB system treating a septic tank effluent 
(Vanier and Dahab, 1997). Gersberg et al. (1989) found a 
two log reduction in total coliforms in a VSB system treat¬ 
ing primary effluent. The coliform removal in two VSB sys¬ 
tems in England treating secondary effluents varied be¬ 
tween 40% and 99%, but effluent values did not meet dis¬ 
charge requirements (Griffin et al., 1998). Fecal coliform 
reductions were typically two logs (1 x 10 6 to 1 x 10 4 /1 OOmL) 
in several experimental VSB systems in Tennessee, ex¬ 
cept for two cells operated in a fill and drain mode. These 
fill and drain cells achieved a three log reduction with the 


same influent wastewater (George et al., 2000). For de¬ 
sign purposes a two log reduction is a reasonable esti¬ 
mate of VSB performance. 

5.5 Design Considerations 
5.5 .1 Media Size and Hardness 

The media of a VSB system perform several functions; 
they (1) are rooting material for vegetation, (2) help to 
evenly distribute/collect flow at the inlet/outlet, (3) provide 
surface area for microbial growth, and (4) filter and trap 
particles. For successful plant establishment, the upper¬ 
most layer of media should be conducive to root growth. A 
variety of media sizes and materials have been tried, but 
there is no clear evidence that points to a single size or 
type of media, except that the media should be large 
enough that it will not settle into the void spaces of the 
underlying layer. It is recommended that the planting me¬ 
dia not exceed 20 mm (3/4 in) in diameter, and the mini¬ 
mum depth should be 100 mm (4 in). 

The media in the inlet and outlet zones (see Figure 5- 
11) should be between 40 and 80 mm (1.5-3 in) in diam¬ 
eter to minimize clogging and should extend from the top 
to the bottom of the system. The inlet zone should be about 
2 m long and the outlet zone should be about 1 m long. 
These zones with larger media will help to even distribute 
or collect the flow without clogging. The use of gabions 
(wire rock baskets used for bank stabilization) to contain 
the larger media simplifies construction. Gabions may also 
make it easier to remove and clean the inlet zone media if 
it becomes clogged. 

Any portion of the media that is wetted is a surface on 
which microbes grow and solids settle and/or accumulate. 
Media in VSBs have ranged from soil to 100 mm (4 in.) 
rock. Experience with soil and sand media shows that it is 
very susceptible to clogging and surfacing of flows, even if 
influent TSS concentrations are minimal, so soil or sand 
media should be avoided. Gravel and rock media have 
been used successfully, with smaller diameter media be¬ 
ing more susceptible to clogging, and larger media more 
difficult to handle during construction or maintenance. 
Crushed limestone can be used, but is not recommended 
for VSB systems because of the potential for media breakup 
and dissolution under the strongly reducing environment 
of a VSB, which can lead to clogging. Media with high iron 
or aluminum will have more sites for phosphorus binding 
and should enhance phosphorus removal, but only during 
the first few months of operation. The limited removal ca¬ 
pability is probably not worth an added expense if it is not 
available locally at a reasonable cost. Alternative media 
such as shredded tires, plastic trickling filter media, ex¬ 
panded clay aggregates and shale with potentially high 
phosphorus absorptive capacity have been used, but there 
is inadequate data to make a recommendation for or 
against their use. 

There does not appear to be a clear advantage in pollut¬ 
ant removal with different sized media in the 10 to 60 mm 
(3/8 - 2 in.) range. Therefore, it is recommended that the 


101 






Media Surface 



; 

k 


E 

CD 


O 


/ 

f 


Inlet 

Zone 

Treatment Zone 

Outlet 

Zone 

Zone 1 

Zone 2 



2 m 

30% of Length 

70% of Length 

1 m 


Outlet 


Side View 


Figure 5-11. Proposed Zones in a VSB 


average diameter of the treatment zone media be between 
20 and 30 mm (3/4 - 1 in.) in diameter as a compromise 
between the potential for clogging and ease of handling. 
To minimize settling of the media smooth, rounded media 
with a Mohs hardness of 3 or higher is recommended if it 
is available locally at a reasonable cost. Based on the data 
in Table 5-1, the hydraulic conductivity of the 20 - 30 mm 
diameter clean media is assumed to be 100,000 m/d. 

5.5.2 Slopes 

The top surface of the media should be level or nearly 
level for easier planting and routine maintenance. Theo¬ 
retically, the bottom slope should match the slope of the 
water level to maintain a uniform water depth throughout 
the VSB. However, because the hydraulic conductivity of 
the media varies with time and location, it is not practical 
to determine the bottom slope this way, and the bottom 
slope should be designed only for draining the system, 
and not to supplement the hydraulic conductivity of the 
VSB. A practical approach is to uniformly slope the bottom 
along the direction of flow from inlet to outlet to allow for 
easy draining when maintenance in required. No research 
has been done to determine an optimum slope, but a slope 
of 1/2 to 1% is recommended for ease of construction and 
proper draining (Chalk & Wheale, 1989). Care should be 
taking when grading the bottom slope to eliminate low 
spots, channels and side-to-side sloping which will pro¬ 
mote dead volume or short-circuiting. 

The slope of the berms containing a VSB should be as 
steep as possible, consistent with the soils, construction 
methods and materials. Shallow side slopes create larger 
areas which capture and route precipitation into the VSB, 


which may be detrimental to system performance. Also, 
the site should be graded to keep off-site runoff out of the 
VSB. 

5.5.3 Inlet and Outlet Piping 

The inlet piping must be designed to minimize the po¬ 
tential for short-circuiting and clogging in the media, and 
maximize even flow distribution. For VSBs with length-width 
ratios less than one, additional care must be taken to spread 
the influent across the whole width of the VSB. Standard 
hydraulic design principles and structures (e.g. adjustable 
weirs and orifices) are used to split, balance evenly dis¬ 
tribute flows (WEF, 1998). The recommended method to 
evenly distribute flows is to use reducing tees or 90 de¬ 
gree elbows which can be rotated on the header (see Chap¬ 
ter 6). The main advantage of a rotating fitting is that it 
allows the operator to easily adjust the distribution of the 
influent, which may help in reducing media clogging. When 
the potential for public access exists, a cover over the in¬ 
fluent distribution system must be used. Possible covers 
include half sections of pipe or cavity chambers, as used 
in leach fields. If piping with orifices is used to distribute 
flows instead of a pipe with rotating fittings, it is necessary 
to minimize the headloss in the distribution piping so that 
the headloss through the orifices controls the flow. This 
requirement limits the number and size of orifices used, 
and makes the distribution piping large enough so that the 
velocity in it is low. The orifices should be evenly spaced 
at a distance approximately equal to 10% of the cell width. 
For example, a system 20 m (65 ft) wide should have ori¬ 
fices placed every 2 m (6.5 ft). If poor design causes waste- 
water to always discharge through only some of the ori¬ 
fices, clogging of the media or accumulation of a surface 


102 






































layer of solids near those orifices can become a problem, 
especially for an influent with relatively high suspended 
solids, such as pond effluent. Finally, the inlet piping should 
be designed to allow for inspection and clean-out by the 
operator. 

The outlet piping must be designed to minimize the po¬ 
tential for short-circuiting, to maximize even flow collec¬ 
tion, and to allow the operator to vary the operating water 
level and drain the bed. For VSBs with length-width ratios 
less than one, additional care must be taken to collect the 
influent from the whole width of the VSB. A collection header 
with orifices that is placed across the entire width of the 
bottom of the VSB is recommended to promote even flow. 
The collection header should be designed with the same 
hydraulic principles used for inlet distribution piping. Slot¬ 
ted or perforated drainage pipe can be used if the collec¬ 
tion header is not too long, but properly sized and spaced 
orifices in a large diameter collection header allow a de¬ 
signer to use a longer collection header and still achieve 
balanced flow collection. The recommended maximum dis¬ 
tance between orifices in the collection header is 10% of 
the cell width. The relative potential for clogging with slot¬ 
ted or perforated drainage pipe versus a longer collection 
header with fewer orifices is unknown. Finally, the outlet 
piping should be designed to allow for clean-out by the 
operator. 

A simple device to adjust the water level in a separate, 
covered, outlet box is recommended to achieve variable 
water level control (see Chapter 6). It is recommended that 
there be only one collection header and adjustable-level 
device per cell of a multiple cell VSB system. The adjust¬ 
able device should allow the operator to flood the VSB to a 
depth of 50 mm (2 in.) above the surface of the media (for 
help in weed control), and to draw-down or drain the cell 
for maintenance. 

5.5.4 System Depth, Width and Length 

The impact of water depth on pollutant removal is not 
clear. One problem with almost all published information 
on VSB systems is that even though the media depth may 
be known, the actual operating water level is not known. 
The TTU study found slightly better BOD removal with 
greater media depth, when comparing 45 cm (18 in) with 
30 cm (12 in) systems operated at same areal loading, but 
it is unclear if this was due only to the increased HRT. No 
other study has tested this result or determined the opti¬ 
mum depth for a VSB system (George et al., 2000). One 
study suggested that total root penetration of the media 
was critical to pollutant removal and recommended that 
system depth be set equal to the maximum root depth of 
the wetland species to be used in the VSB (Gersberg et al. 
1983). However, as discussed previously, plants supplied 
with abundant nutrients near the surface will not neces¬ 
sarily grow roots to their maximum depth. As a safety fac¬ 
tor Kadlec and Knight (1996) recommend allowing room 
for solids accumulation in the bottom of the VSB, but the 
need for this has not been proven. Typical average media 
depths in VSB systems have ranged from 0.3 to 0.7 m (12 


to 28 in.), and various researchers have recommended 
depths from 0.4 to 0.6 m (16 to 24 in.). 

As discussed previously there is evidence for preferen¬ 
tial flow below the root zone through media with a higher 
conductivity. In order to minimize this flow, a shallower 
depth would be required. On the other hand, a shallower 
depth may require a greater area to achieve a desired HRT. 
Until future studies provide better information on optimum 
water depth, it is recommended to use a design maximum 
water depth (at the inlet of the VSB) of 0.40 m (16 in.). The 
depth of the media will be defined by the level of the waste- 
water at the inlet and should be about 0.1 m (4 in.) deeper 
than the water. 

The overall width of a treatment system using VSBs is 
defined by Darcy’s Law, which is a function of the flow, 
ALR, water depth and hydraulic conductivity. The width of 
a individual VSB is set by the ability of the inlet and outlet 
structures to uniformly distribute and collect the flow with¬ 
out inducing short-circuiting. The recommended maximum 
width in a TVA design manual is 61 m (200 ft.). If the de¬ 
sign produces a larger value, the user should divide the 
VSB into several cells that do not exceed 61 m in width. As 
discussed previously, several researchers have noted that 
most BOD and TSS is removed in the first few meters of a 
VSB, but some recommend minimum lengths ranging from 
12 to 30 m (40 to 100 ft) to prevent short-circuiting. The 
recommended minimum length for this manual is 15 m (50 
ft). 

Although much has been made of the aspect (length- 
width) ratio in early constructed wetlands literature, the only 
prerequisite for treatment is the area as defined by the 
ALR. A study by Bounds, et.al. (1998) found that there was 
no significant difference in TSS or CBOD removal in three 
parallel VSB systems with aspect ratios of 4:1, 10:1, and 
30:1. In all three systems the majority of TSS and CBOD 
was removed in the first third on the VSB. Removals were 
also unaffected by stressing the systems with large hy¬ 
draulic spikes and intermittent loading. The TTU study also 
found no significant difference in systems with 1:4 and 4:1 
aspect ratios (George et al., 2000). Therefore, the aspect 
ratio is not a factor in the overall design. However, the rec¬ 
ommended values for maximum width and minimum length 
discussed previously will tend to result in individual VSB 
cells with an length-width ratio between 1:1 and 1:2. 

5.6 Design Example for a VSB Treating 
Septic Tank or Primary Effluent 

The design has two basic assumptions. First, the total 
VSB has four zones (see Figure 5-11). The inlet and outlet 
zones were discussed in section 5.5.1. Based on the lit¬ 
erature as discussed previously, the initial treatment zone 
will (1) occupy about 30% of the total area, (2) perform 
most of the treatment, and (3) have a big decrease in hy¬ 
draulic conductivity (use K = 1 % of clean K). The final treat¬ 
ment zone will occupy the remaining 70% of the area and 
have little change in hydraulic conductivity (use K = 10% 
of clean K). The second basic assumption is that Darcy’s 


103 






Law, while not exact, it is good enough for design pur¬ 
poses. The sizing of the initial and final treatment zones 
follows these steps: 

1) determine the surface area, using recommended 
ALR 

2) determine the width, using Darcy’s Law 

3) determine the length and headloss of the initial treat¬ 
ment zone, using Darcy’s Law 

4) determine the length and headloss of the final treat¬ 
ment zone, using Darcy’s Law 

5) determine bottom elevations, using bottom slope 

6) determine water elevations throughout the VSB, 
using headloss 

7) determine water depths, accounting for bottom slope 
and headloss 

8) determine required media depth 

9) determine the number of VSB cells 

For this example the following values are given: 

• Maximum Monthly Flow (Q) = 200 m 3 /d 

• Maximum Monthly Influent (CO) BOD = 100 mg/L = 
100 g/m 3 

• Maximum Monthly Influent (CO) TSS = 100 mg/L = 100 
g/m 3 

• Required discharge limits = 30 mg/L BOD and TSS 
Recommended values for VSBs (see Table 5-4) are: 

• ALR for BOD = 6 g/m 2 -d 

• ALR for TSS = 20 g/m 2 -d 

• Use washed, rounded media 20-30 mm in diameter, 
clean K = 100,000 m/d 

• Hydraulic conductivity of initial treatment zone (K) = 
1 % of 100,000 = 1000 m/d 

• Hydraulic conductivity of final treatment zone (K ( ) = 
10% of 100,000 = 10,000 m/d 

• Bottom slope (s) = _% = 0.005 

• Design water depth at inlet (D w0 ) = 0.4 m 

• Design water depth at beginning of final treatment zone 
(D J = 0.4 m 

• Design media depth (D m ) = 0.6 m 

• Maximum allowable headloss through initial treatment 
zone (dti) = 10% of D m = 0.06 m 


5.6.1 Determine the Surface Area (As) for 
Both Pollutants 

A=(Q)(C 0 )/ALR 

For BOD, A s = (200 m 3 /d)(100 g/m 3 ) / 6 g/m 2 -d = 3333 m 2 

For TSS, A s = (200 m 3 /d)(100 g/m 3 ) / 20 g/m 2 -d = 1000 m 2 

Use the larger area requirement, or 3333 m 2 . 

The surface area for the initial treatment zone (A sj ) = 
(30%) (3333 m 2 ) = 1000 m 2 

The surface area for the final treatment zone (A s( ) = 
(70%) (3333 m 2 ) = 2333 m 2 

5.6.2 Determine the Width 

Determine the minimum width (W) needed to keep the 
flow below the surface, using Darcy’s Law (Eq. 5-1) and 
recommended values for the initial treatment zone. 

Q = (K)(W)(D w0 )(dh/L) 

where: L = length of initial treatment zone = (A sj ) / (W) 
Substitute and rearrange equation to solve for W: 

W 2 = (Q)(A si ) / (K)(dh.)(D w0 ) (5-3) 

For this example: 

W 2 = (200 m 3 /d)(1000 m 2 ) / (1000 m/d)(0.06 m)(0.4 m) 
= 8333 m 2 
W =91.3 m 

This is the width for which the headloss equals 0.06 m, 
given all the parameters as defined. The designer must 
use a width equal to or greater than this to ensure that the 
headloss is less than or equal to the design value. 

5.6.3 Determine the Length and Headloss 
(Eq. 5-2) of the Initial Treatment 
Zone (L) 

L = (A sj ) / (W) = (1000 m 2 ) / (91.3 m) = 11.0 m 

This is the length for which the headloss equals 0.06 m, 
given all the parameters as defined. The designer must 
use a length less than or equal to this to ensure that the 
headloss is less than or equal to the recommended value. 

dh j= (Q)(L) / (K)(W)(DJ = (200 m 3 /d)(11.0 m) / (1000 
m/d)(91.3 m)(0.4 m) = 0.06 m 

5.6.4 Determine the Length and Headloss 
of the Final Treatment Zone (L) 

L, = (A,) / (W) = (2333 m 2 ) / (91.3 m) = 25.6 m 

This is the length where the total area of the VSB will be 
exactly equal to the value set by the ALR. The designer 
must use a length equal to or greater than this to ensure 


104 







that the surface area is equal to or greater than the recom¬ 
mended value. 

dh,= (Q)(L f ) / (K f )(W)(D wf ) = (200 m 3 /d)(25.6 m) / 

(10,000 m/d)(91.3 m)(0.4 m) = 0.01 m 

5.6.5 Determine Bottom Elevations 

E be = elevation of bottom at outlet = 0 (reference point 
for all elevations) 

E bf = elevation of bottom at beginning of final treatment 
zone = (s)(L f ) = (0.005)(25.6 m) = 0.13 m 

E b0 = elevation of bottom at inlet = (s)(L + L ( ) = 
(0.005)(11.0 m +25.6 m) = 0.18 m 

5.6.6 Determine the Water Surface 
Elevations 

E^ = elevation of water surface at beginning of final 
treatment zone 

= E bf + = 0.13 m + 0.4 m = 0.53 m (D^ = 0.4 m 

was an initial recommended value) 

E a = elevation of water surface at outlet = E - dh = 

we wf f 

0.53 m - 0.01 m = 0.52 m 

E = elevation of water surface at inlet = E . + dh = 

w0 wf i 

0.53 m + 0.06 m = 0.59 m 

5.6.7 Determine Water Depths 

D w0 = depth of water at inlet = E w0 - E b0 = 0.59 m - 0.18 
m = 0.41 m (about equal uTdesign D w0 , so okay.) 

D wf = depth of water at beginning of final treatment 
zone 

= E^ - E bf = 0.53 m - 0.13 m = 0.40 m (equal to 
design D^, so okay.) 

D we = depth of water at outlet = E we - E be = 0.52 m - 0 = 
we 0.52 m 

5.6.8 Determine the Media Depth 

The media depth will depend on whether the designer 
wants a level media surface, or a minimum depth-to-water 
(DJ throughout the VSB. 

a) If a level surface is desired, the elevation must be 
greater than the highest water elevation, which is at the 
inlet, E _ = 0.59 m. A media elevation set at 0.65 m would 
be reasonable, and the following media depths and Dtw’s 
result: 

D m0 = depth of media at inlet = 0.65 m - E b0 = 0.65 m - 
0.18 m = 0.47 m 

D = depth of media at beginning of final treatment 
zone = 0.65 m - E bf = 0.65 m - 0.13 m = 0.52 m 


D me = depth of media at outlet = 0.65 m - 0 = 0.65 m 

D tw0 = depth-to-water at inlet = 0.65 m - E w0 = 0.65 m - 
0.59 m = 0.06 m 

D tw( = depth-to-water at beginning of final treatment 
zone = 0.65 m - E . = 0.65 m - 0.53 m = 0.12 m 

wf 

D, a = depth-to-water at outlet = 0.65 m - E,= 0.65 m 

two 1 we 

- 0.52 m = 0.13 m 

The depth-to-water is small at the inlet (0.06 m) and the 
designer may want to add an additional layer of media in 
the first few meters of the initial treatment zone as an added 
precaution against surfacing, even though the design ALR 
and K values is very conservative. The resulting D tw in the 
final treatment zone would be 0.12 to 0.13 m, which should 
not inhibit the growth of aquatic species. 

b) If a constant depth-to-water throughout the VSB is 
desired (e.g. 0.1 m), then the media depth would be calcu¬ 
lated as follows: 

E m0 = elevation of media surface at inlet = E w0 + 0.1 m 
= 0.59 m + 0.1 m = 0.69 m 

E mf = elevation of media surface at beginning of final 
treatment zone 

= E . + 0.1 m = 0.53 m + 0.1 m = 0.63 m 

w( 

E = elevation of media surface at outlet = E +0.1 

me we 

m = 0.52 m + 0.1 m = 0.62 m 

D m0 = depth of media at inlet = E m0 - E b0 = 0.69 m - 
0.18 m = 0.51 m 

D mf = depth of media at beginning of final treatment 
zone = E - E = 0.63 m - 0.13 m = 0.56 m 

mf bf 

D = depth of media at outlet = E - 0 = 0.52 m - 0 = 
0.52 m 

This approach would result in a drop in the media sur¬ 
face of (0.69 m - 0.51 m) of 0.18 m over the 11.0 m length 
of the initial treatment zone (slope = 1.6%), which would 
probably not impair operation and maintenance activities. 

5.6.9 Determine Number of VSB Cells 

It is recommended that at least two VSBs be used in 
parallel in all but the smallest systems, so that one of the 
VSBs can be taken out of service for maintenance or re¬ 
pairs without causing serious water quality violations. In 
this example, the total size of the VSB system is 91.3 m 
wide by 36.6 m long. Therefore, use two VSBs, each 46 m 
wide and 37 m long could be used. Other combinations of 
length and width that have the required surface area will 
also work as long as the hydraulics conditions are meet. 
Also remember that inlet and outlet zones will add to the 
overall length of the VSB. 


105 






Table 5-4. Summary of VSB Design Guidance. 

Recommended for use after primary 
sedimentation (e.g. septic tank, Imhoff 
tank, primary clarifier) VSBs not 
recommended for use after ponds 
Pretreatment because of problems with algae 


Surface Area 

Based on desired effluent quality and areal loading rates as follows: 


BOD 

BOD 

TSS 

TKN 

TP 


6 g/m 2 -d (53.5 Ib/ac-d) to attain 30 mg/L 
effluent 

1.6 g/m 2 -d (14.3 Ib/ac-d) to attain 20 mg/L 
effluent 

20 g/m2-d (178 Ib/ac-d) to attain 30 mg/L 
effluent 

Use another treatment process in 

conjunction with VSB 

VSBs not recommended for phosphorus 

removal 


Depth 

Media (typical) 0.5 - 0.6 m (20 - 24 in.) 

Water (typical) 0.4 - 0.5 m (16 - 20 in.) 


coming the inadequate purification capacity of certain soils. 
They view VSBs as passive systems with low operation 
and maintenance requirements. 

A review of various on-site VSB design guidelines by 
Mankin and Powell (1998) revealed that there was a large 
variation among the designs. Recommended depth var¬ 
ied from 0.3 to 0.8 m (1 to 2.5 ft), with the great majority 
between 0.3 and 0.5 m (1 and 1.5 ft). For a three bedroom 
house typical VSB areas varied from < 10 m 2 to > 100 m 2 
(104 to 1088 ft 2 ), HRTs varied from 1.3 to 6.5 d, and length- 
width ratio varied from 71:1 to 1.8:1. Median values were 
a depth of 0.45 m (1.5 ft), area of 30 m 2 (315 ft 2 ), and HRT 
of 4.7 d. Gravel size guidelines varied from 0.65 cm (0.25 
in.) to 7.5 cm (3.0 in.) 

These authors sampled three typical VSB systems in 
Kansas and compared them with other reported data. De¬ 
spite employing a larger than average area, the units failed 
to meet a 30/30 BOD/TSS requirement at two of the three 
sites. 


Length 

Width 


Bottom slope 


As calculated (see design example); 
minimum of 15 m (49 ft) 

As calculated (see design example); 
maximum of 61 m (200 ft) 


0.5- 1% 


Top slope 


level or nearly level 


Hydraulic Conductivity 

First 30% of length 1% of clean K 
Last 70% of length 10% of clean K 


Given the general lack of operation and maintenance 
requirements and the potential aesthetic appearance of 
VSB systems, their attractiveness to local and state regu¬ 
lators is quite predictable. As a passive system potentially 
capable of meeting a 30/30 BOD/TSS requirement, they 
have obvious advantages over mechanical systems which 
require a significant management program and electrical 
support to function satisfactorily. Also, their general reli¬ 
ability, when compared to mechanical systems, offers ad¬ 
ditional protection against clogging of the soil’s infiltrative 
surface. 


Media 

All media should be washed clean of fines and debris; more rounded 
media will generally have more void spaces; media should be resistant 
to crushing or breakage. 

Inlet zone 40 - 80 mm (1.5 - 3.0 in) 

(1st 2 m (6.5 ft)] 

Treatment zone 20 - 30 mm (3/4 -1 in) [use clean K = 

100,000, if actual K not known] 


Outlet zone 40 - 80 mm (1.5 - 3.0 in) 

[last 1 m (3.2 ft)] 

Planting media 5 - 20 mm (1/4 - 3/4 in) 

[top 10 cm (4 in.)] 


Miscellaneous Use at least 2 VSBs in parallel 

Use adjustable inlet device with 
capability to balance flows 
Use adjustable outlet control device with 
capability to flood and drain system 


5.7 On-site Applications 

A number of states, including Louisiana, Kentucky, Kan¬ 
sas, Arkansas, Texas and Indiana, have used VSBs for 
on-site wastewater management. Kentucky alone lists over 
4000 such installations (Thom et al., 1998). Most of these 
states have adopted VSBs as a pretreatment step prior to 
soil infiltration in an effort to protect groundwater by over- 


Based on the VSB design guidance presented previously, 
a three bedroom home would require a VSB of 100 m 2 , 
assuming six persons and a BOD loading of 100 g/cap-d. 
As expected, due to the conservative nature of the design 
approach presented in this chapter, this area is at the high 
end of the areas found by Mankin and Powell (1998). 

There should be minimal deviation from the recommen¬ 
dations of Table 5-4, except that simplified inlet and outlet 
configurations, appropriate for small on-site systems, can 
be used. As with larger VSBs, some means of post-aera¬ 
tion and disinfection will be required if surface discharge is 
contemplated. Discharge to soil infiltration is more likely, 
and soil absorption guidelines provided by the State will 
apply. 

5.8 Alternative VSB Systems 

Alternative VSB systems are those that operate with 
some schedule of filling and draining the media. Fill and 
drain VSB systems are similar to sequencing batch reac¬ 
tors, intermittent sand filters, or overland flow systems in 
that the flow into a single cell of a system is intermittent. 
Draining a VSB system is a simple way to introduce more 
oxygen into the media. Clearly plants are playing a lesser 
role in these systems and they are inherently quite differ¬ 
ent from a natural wetland. Nevertheless they are discussed 
here because they have been identified as constructed 


106 






wetlands and have evolved from conventional VSB sys¬ 
tems. They are more complex to operate than a conven¬ 
tional VSB system. 

One of the first and more unconventional alternative VSB 
systems was developed in England and tested in England 
and Egypt in the late 1980s (May et al., 1990). The sys¬ 
tem, called a gravel bed hydroponic system, is very simi¬ 
lar to an overland flow process except that the wastewater 
flows through 8-10 cm (3-4 in) of gravel. Loading is typi¬ 
cally intermittent except when denitrification is desired. 

Researchers at TTU (1999) experimented with alternat¬ 
ing fill and drain VSB wetlands for one year in 1994 and 
compared the results to conventional VSBs in side by side 
testing. The alternating fill and drain cells followed a con¬ 
ventional VSB cell. Effluent from the fill and drain cells was 
recycled back to the conventional cell. They found some 
improvement in nitrogen removal but overall the results 
were not as good as they had expected. 

Researchers at TVA have developed (and patented) a 
system in which the wastewater is quickly drained from 
one wetland cell and pumped into a second parallel cell 
(Behrends, et al., 1996). The draining and filling occurs 
within 2 hours and then the process is reversed; the sec¬ 
ond cell is drained quickly and the first cell is refilled. The 
reciprocating flow process is repeated continuously, with 
a small amount of influent continually added to the first 
cell and a fraction of the wastewater continually drawn from 
the second cell as effluent. The reciprocating two cell sys¬ 
tem was compared with a conventional two cell system for 
six months in side by side testing in late 1995 and early 
1996 at Benton, Tennessee. Continued operation of both 
two-cell pairs in the reciprocating mode has continued since 
May of 1996. Comparing conventional operation to the 
reciprocating mode, the reciprocating mode produced sig¬ 
nificantly lower effluent BOD and ammonia nitrogen. 

One of the most studied full-scale VSB systems is lo¬ 
cated at the Village of Minoa in New York State (see Chap¬ 
ter 9). Two New York State agencies and the USEPA pro¬ 
vided grant funds to the Village for incorporation of sev¬ 
eral special features in the VSB system and for a research 
and technology transfer study of the system by research¬ 
ers at Clarkson University, Potsdam, NY. The system was 
originally designed and operated as a conventional VSB 
system, but during 15 months treating a primary effluent, 
the system performed very poorly compared to its design 
expectations. Faced with numerous complaints from 
nearby residents about hydrogen sulfide odors, the opera¬ 
tors started operating the system with occasional draw¬ 
down periods to control odors. The drawdown significantly 
reduced odors. In April 1997, when the experimental plan 
for the system called for the three cells to operated in se¬ 
ries, the Minoa operators decided to increase the flow by 
100% and change the operation to a fill and drain mode. 
The fill and drain operation included a resting period dur¬ 
ing the drained condition and continuous operation for some 
time after filling. The fill and drain operation eliminated the 
hydrogen sulfide odors and also resulted in a significant 


improvement in the effluent quality. However, the opera¬ 
tors were not satisfied with the improved performance and 
experimented further. In 1998 they changed the operation 
to a mode that continued to the writing of this manual. Two 
of the wetland cells operate in a parallel fill and drain mode 
very similar to sequencing batch reactors. The third cell is 
operated in a conventional mode but in series with the first 
two cells. This mode of operation has resulted in an addi¬ 
tional significant improvement in effluent quality over the 
previous fill and drain mode of operation. See section 9.8 
for a more detailed description of the operation and the 
results. 

Within the last five years, several unsaturated vertical 
flow systems have been constructed and tested in Europe. 
Most have been used for tertiary treatment of secondary 
effluents but they have also been used for treating septic 
tank and sugar beet processing effluents. They appear to 
perform significantly better than conventional VSB systems. 
Recommended design loadings are approximately twice 
that for conventional VSB systems. 

5.9 References 

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Bavor, H.J., D.J. Roser, RJ. Fisher, and I.C. Smalls. 1989. 
Performance of solid-matrix wetland systems viewed as 
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Wetlands for Wastewater Treatment. Chelsea, Ml: Lewis 
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Bavor, H.J. and T.J. Schulz. 1993. Sustainable suspended 
solids and nutrient removal in large-scale, solid matrix, 
constructed wetland systems. In: G.A. Moshiri (ed.) Con¬ 
structed wetlands for water quality improvement. Boca 
Raton, FLLewis Publishers, pp. 219-225. 

Behrends, L.L., Coonrod, H.S., Bailey E. and M.J. Bulls. 1993. 
Oxygen Diffusion Rates in Reciprocating Rock Biofilters: 
Potential Applications for Subsurface Flow Constructed 
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Wetlands Conference, August 16-17, 1993, University 
of Texas at El Paso. 

Behrends, L.L., F. J. Sikora, H.S. Coonrod, E. Bailey and C. 
McDonald. 1996. Reciprocating Subsurface-Flow Con¬ 
structed Wetlands for Removing Ammonia, Nitrate, and 
Chemical Oxygen Demand: Potential for Treating Do¬ 
mestic, Industrial and Agricultural Wastewater. Vol 5, Pp 
251-263. In: Proceedings of the Water Environment 
Federation 69th Annual Conference. Dallas, TX. 

Bhattarai, R.R. and D.M. Griffin, Jr. 1998. Results of tracer 
tests in rock plant filters. Department of Civil Engineer¬ 
ing, Louisiana Tech University, Ruston, LA. 

Bounds, H.C., J. Collins, Z. Liu, Z. Qin, andT.A. Sasek. 1998. 
Effects of length-width ratio and stress on rock-plant fil¬ 
ter operation. Small Flow Journal, 4(1 ):4-14. 


107 







Bowmer, K.H. 1987. Nutrient removal from effluents by 
an artificial wetland: influence of rhizosphere aera¬ 
tion and preferential flow studied using bromide and 
dye tracers. Water Research, 21 (5):591-599. 

Breen, P.F. and A.J. Chick. 1995. Rootzone dynamics 
in constructed wetlands receiving wastewater: a 
comparison of vertical and horizontal flow systems. 
Water Science & Technology, 32(3):281-290. 

Chalk, E. and G. Wheale. 1989. The root-zone process 
at Holtby Sewage Treatment Works. Journal IWEM, 
3:201-207. 

Cooper, P.F. 1990. European Design and Operations 
Guidelines for Reed Bed Treatment Systems, Rep. 
UI17, Water Research Centre, Swindon, U.K. 

Cooper, P.F., J.A. Hobson and S. Jones. 1989. Sewage 
Treatment by Reed Bed Systems. Journal of the In¬ 
stitution of Water and Environmental Management. 
3 (1) 60. 

Crites, R. and G.Tchobanoglous. 1998. Small and de¬ 
centralized wastewater management systems. San 
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Dahab, M.F. and R.Y. Surampalli. 1999. Predicting Sub¬ 
surface Flow constructed Wetlands Performance: A 
Comparison of Common Design Models. In: Pro¬ 
ceedings of the Water Environment Federation 72th 
Annual Conference. New Orleans, LA. 

DeShon, G.C., A.L. Thompson, and D.M. Sievers. 1995. 
Hydraulic properties and relationships for the de¬ 
sign of subsurface flow wetlands. Presented at Ver¬ 
satility of Wetlands in the Agricultural Landscape 
Conference, Tampa, FL, Sept. 17-20, 1995. 

Fisher, P.J. 1990. Hydraulic characteristics of con¬ 
structed wetlands at Richmond, NSW, Australia. In: 
P.F. Cooper and B.C. Findlater (eds.) Constructed 
Wetlands in Water Pollution Control. Oxford, UK: 
Pergamon Press, pp. 21-31. 

Gearheart, R.A. 1998. Use of FWS constructed wetlands 
as an alternative process treatment train to meet 
unrestricted water reclamation standards. Presented 
at AWT-98, Advanced Wastewater Treatment, Re¬ 
cycling and Reuse, Milan, Italy, pp. 559-567. 

Gearheart, R.A. et al. 1999. Free water surface wet¬ 
lands for wastewater treatment: a technology as¬ 
sessment. USEPA, Office of Water Management, US 
Bureau of Reclamation, City of Phoenix, AZ. 

George, D.B. et al. 2000. Development of guidelines 
and design equations for subsurface flow con¬ 
structed wetlands treating municipal wastewater. 
USEPA, Office of Research and Development, Cin¬ 
cinnati, OH. 


Gersberg, R.M., B.V. Elkins and C.R. Goldman. 1983. 
Nitrogen Removal in Artificial Wetlands. Water Re¬ 
search 17 (9) 1009. 

Gersberg, R.M. et al. 1984. The Removal of Heavy Met¬ 
als by Artificial Wetlands, In: Proc Water Reuse 
Symp. Ill, Vol 2, AWWA Research Foundation, 639. 

Gersberg, R.M., et al. 1986. Role of Aquatic Plants in 
Wastewater Treatment by Artificial Wetlands. Wa¬ 
ter Research 20 (3) 363. 

Gersberg, R.M., Gearheart, R.A., and M. Ives. 1989. 
Pathogen Removal in Constructed Wetlands, Proc. 
From First International Conference on Wetlands for 
Wastewater Treatment, Chattanooga, TN, June 
1988, Ann Arbor Press. 

Green, M.B. and J. Upton. 1994. Constructed Reed 
Beds: A Cost-Effective Way to Polish Wastewater 
Effluents for Small Communities. Water Env. Res. 
66 (3) 188. 

Griffin, P., B. Green and A.Pritchard. 1998. Pathogen 
Removal in Subsurface Flow Constructed Reed 
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Federation 71st Annual Conference. Orlando, FL. 

Heukelekian, H. and J.L. Balmat. 1959. Chemical com¬ 
position of the particulate fractions of domestic sew¬ 
age. Sewage & Industrial Wastes, 81:413-423. 

Jenssen, P.T. M. Muehlan, and T. Kregstad. 1993. Po¬ 
tential use of constructed wetlands for wastewater 
treatment in northern environments. In: Proceedings 
of 2nd International Conference on Design and Op¬ 
eration of Small Wastewater Treatment Plants, pp. 
193-200. 

Kadlec, R.H. and R.L. Knight. 1996. Treatment Wet¬ 
lands. Boca Raton, FL: Lewis-CRC Press. 

Kadlec, R.H. and J.T. Watson. 1993. Hydraulics and sol¬ 
ids accumulation in a gravel bed treatment wetland. 
In: G.A. Moshiri (ed.) Constructed wetlands for wa¬ 
ter quality improvement. Boca Raton, FL:Lewis Pub¬ 
lishers, pp. 227-235. 

Kickuth, R. 1981. Abwasserreinigung in mosaikmatrizen 
aus aeroben und anaerobenteilbezirken. In: F. 
Moser (Ed), Grundlagen der Abwassereinigung, pp 
639-665. 

King, A.C., C.A. Mitchell, and T. Howes. 1997. Hydrau¬ 
lic tracer studies in a pilot scale subsurface flow con¬ 
structed wetland. Water Science & Technology, 
35(5): 189-196. 

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of high nitrogen landfill leachate. Project Number 
94-IRM-U, Water Environment Research Founda¬ 
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1992. Predicting water mounding in subsurface rock 
bed wetlands. Presented at the Mid-Central Con¬ 
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gineers, St. Joseph, MO, March 13-14, 1992. 

Mankin, K.R. and G.M. Powell. 1998. Onsite rock-plant 
filter monitoring and evaluation in Kansas. In: Pro¬ 
ceedings of 8th National Symposium on Individual 
and Small Community Sewage Systems, ASAE, St. 
Joseph, Ml. 

May, E., J.E. Bulter, M.G. Ford, R.F. Ashworth, J.S. Wil¬ 
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Chemical and Microbiological Processes in Gravel 
Bed Hydroponic (GBH) Systems for Sewerage 
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lution Control. Cooper and Findlater (Eds) 
Pergamon Press U.K. 

Netter, R. and W. Bischofsberger. 1990. Hydraulic in¬ 
vestigations on planted soil filters. In: P.F. Cooper 
and B.C. Findlater (eds.) Constructed Wetlands in 
Water Pollution Control. Oxford, UK: Pergamon 
Press, pp. 11-20. 

Netter, R. 1994. Flow characteristics of planted soil fil¬ 
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treatment. In: Proceedings of 7th European Sew¬ 
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Peverly, J.H., J.M. Surface and T. Wang. 1995. Growth 
and Trace Metal Absorption by Phragmites austra¬ 
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Treatment. Ecological Engineering 5, 21. 

Rash, J.K. and S.K. Liehr. 1999. Flow pattern analysis 
of constructed wetlands treating landfill leachate. 
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Reedy, K.R. and W.F. DeBusk. 1985. Nutrient removal 
potential of selected aquatic macrophytes. J. Envi¬ 
ronmental Quality, 19:261. 

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ment. 2nd Ed. NY: McGraw Hill. 

Reed, S.C. and S. Giarrusso. 1999. Sequencing Op¬ 
eration Provides Aerobic Conditions in a Con¬ 
structed Wetland. In: Proceedings of the Water En¬ 
vironment Federation 72th Annual Conference. New 
Orleans, LA. 

Richard, M. and J. Snyder. 1994. Results of the pilot 
wetlands study at Las Amimas, CO. Report to the 
City of Las Animas, Colorado State University, CO. 

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face, and J.H. Peverly. 1995a. Hydraulic conductiv¬ 


ity of gravel and sand as substrates in rock-reed 
filters. Ecological Engineering, 4:321-336. 

Sanford, W.E., T.S. Steenhuis, J.M. Surface, and J.H. 
Peverly. 1995b. Flow characteristics of rock-reed fil¬ 
ters for treatment of landfill leachate. Ecological Engi¬ 
neering, 5:37-50. 

Sanford, W.E. 1999. Substrate type, flow characteristics, 
and detention times related to landfill leachate treat¬ 
ment efficiency in constructed wetlands. In: G. 
Mulamootil, E.A. McBean, and F. Rovers (eds.) Con¬ 
structed wetlands for the treatment of landfill leachate. 
Boca Raton, FL:Lewis Publishers, pp. 47-56. 

Sapkota, D.P. and H.J. Bavor. 1994. Gravel bed filtration 
as a constructed wetland component for the reduction 
of suspended solids from maturation pond effluent. 
Water Science & Technology, 29(4):55-66. 

Smith, I.D., G.N. Bis, E.R. Lemon and L.R, Rozema. 1997. 
A Thermal Analysis of a Sub-surface, Vertical Flow 
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Stengel, E. and Schultz-Hock, R. 1989. Denitrification in 
artificial wetlands. In: D.A. Hammer (ed.) Constructed 
Wetlands for Wastewater Treatment. Chelsea, Ml: 
Lewis Publishers, pp. 484-492. 

Surface, J.M., J.H. Peverly, T.S. Steenhuis, and W.E. 
Sanford. 1993. Effect of season, substrate composi¬ 
tion, and plant growth on landfill leachate treatment in 
a constructed wetland. In: G.A. Moshiri (ed.) Con¬ 
structed wetlands for water quality improvement. Boca 
Raton, FL:Lewis Publishers, pp. 461-472. 

Tanner, C.C. and J.P. Sukias. 1995. Accumulation of or¬ 
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Tanner, C.C., J.P.S. Sukias, and M.P. Upsdell. 1998. Or¬ 
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bed constructed wetlands treating farm dairy waste- 
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face Flow Constructed Wetlands for Small Commu¬ 
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ing effluent treatment in surface-flow and gravel-bed 


109 




constructed wastewater wetlands. In: P.F. Cooper and 
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Watson, J.T., K.D. Choate, and G.R. Steiner. 1990. Per¬ 
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per and B.C. Findlater (eds.) Constructed Wetlands in 


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versity, NY. 


110 







Chapter 6 

Construction, Start-up, Operation, and Maintenance 


6.1 Introduction 

Constructed wetland systems require infrequent opera¬ 
tion and maintenance activities to achieve performance 
goals if they are designed and constructed properly. This 
chapter discusses construction details, start-up procedures, 
and operation and maintenance activities for both free water 
surface wetlands and vegetated submerged beds. 

6.2 Construction 

Construction of wetland systems primarily involves com¬ 
mon earth moving, excavating, backfilling, and grading. 
Most of the equipment and procedures are the same as 
those employed for construction of lagoons, shallow ponds, 
and similar containment basins. However, there are as¬ 
pects that require special attention to ensure flow through 
the wetland is uniform over the design treatment volume. 
Also, establishment of vegetation is unique to the basin 
construction and not always within the repertoire of con¬ 
struction contractors. It is the intent of this section to pro¬ 
vide guidance on these special and unique aspects of 
wetland construction. 

6.2.1 Basin Construction 

The basic containment structure of constructed wetlands 
consists of berms and liners. The structural and watertight 
integrity of the liner and surrounding berm are critical. Fail¬ 
ure of either will result in loss of water, risk of ground water 
pollution, and possible loss of plants due to the decline of 
the water level in the wetland. 

6.2.1.1 Basin Layout 

The topography of the site will dictate the general shape 
and configuration of the wetland. Constructing the wetland 
on sloping sites with the long axis along the contour will 
minimize the grading requirements. With proper layout, long 
sloping sites can reduce pumping costs by taking advan¬ 
tage of the available fall. 

6.2.1.2 Site Preparation 

Clearing and grubbing, rough grading, and berm con¬ 
struction use the same procedures, techniques, and equip¬ 
ment used for lagoons and conventional water contain¬ 
ment basins. If possible, it is desirable to balance the cut 
and fill on the site to avoid the need for remote borrow pits 


or soil disposal. If agronomic-quality topsoil exists on the 
site, it should be stripped and stockpiled. In the case of 
FWS wetlands, the topsoil can be utilized as the rooting 
medium for the emergent vegetation and revegetation of 
the berm surfaces. A soil-rooting medium is not required 
for VSB systems. 

To meet its performance expectations, it is critically im¬ 
portant for the water to flow uniformly through the entire 
wetland area. Severe short-circuiting of flow can result from 
improper grading or nonuniform subgrade compaction. The 
operating water depth may be 60 cm (2 ft) or less, so ir¬ 
regularities in the bottom surface can induce preferential 
flow paths. Specified tolerances for grading will depend 
on the size of the wetland. A very large FWS wetland of 
several thousand acres cannot afford the effort to fine grade 
to very close tolerances. Therefore, the wetland should be 
subdivided into several smaller cells or the design should 
incorporate a sizing safety factor to compensate for po¬ 
tential short-circuiting. For smaller wetlands of a few hun¬ 
dred hectares or less, it is usually cost effective to specify 
closer grading tolerances. Bottom grades are an impor¬ 
tant consideration when converting existing lagoons to wet¬ 
lands. Because of the design depths in lagoons, careful 
grading of the bottom may not have been required. In many 
cases in which conversions were made without careful 
regrading, significant short-circuiting has occurred that re¬ 
duced the wetland treatment performance. 

Uniform compaction of the subgrade is also important to 
protect the liner integrity from subsequent construction 
activity (i.e., liner placement, soil placement for FWS wet¬ 
lands, gravel placement for VSB systems) and from stress 
when the wetland is filled. The loading on the liner is ap¬ 
proximately 2,200 kg/m 2 (450 lbs/ft 2 ) including the plant 
mass. Short-circuiting of flow through a FWS wetland also 
can result from ruts and low areas in the subgrades. The 
subgrade should be uniformly compacted to the same lev¬ 
els used for native soils in road subgrades. 

Fine grading and compaction of the native subgrade soils 
also depends on the liner requirements. Most wetland cells 
are graded level from side to side and either level or with a 
slight slope in the direction of flow. Wetlands are often con¬ 
structed with a bottom slope of 1% or less which is suffi¬ 
cient to drain the cell if and when maintenance is required. 


Ill 





6.2.1.3 Berms 

Berms in constructed wetlands contain water within spe¬ 
cific flow paths. Exterior berms are designed to prevent 
unregulated flow releases. Interior berms are used to aug¬ 
ment flow distribution. External berms are typically built to 
provide 0.6 to 1 m (2-3 ft) of freeboard with a width at 
least 3 m (10 ft) at the top to permit service vehicle ac¬ 
cess. The amount of freeboard should be enough to con¬ 
tain a given storm rainfall amount. Side slopes should be 
a maximum of 3:1; however, slopes of 2:1 have been used 
for internal side slopes, particularly when liners or erosion 
control blankets are used. Access ramps into each cell of 
the system should be shallow enough for maintenance 
equipment to enter. All berms should be constructed in 
conformance with standard geotechnical considerations, 
for they may be subject to local dam safety regulations. 
Design considerations for internal berms, however, are less 
critical since they are not designed for water containment. 
See Figure 6-1 for typical design features of constructed 
wetland berms. 

Short-circuiting around the edges of cells has been ex¬ 
perienced in some FWS wetlands where vegetation on the 
berm slope is absent. This is a particular problem if syn¬ 


thetic liners are used. The liners do not provide a good 
rooting medium and so may remain bare. The open water 
gap between the berm and the vegetated area in the wet¬ 
land proper provides a preferential flow path. A soil layer 
can be placed on the berm side slope to establish vegeta¬ 
tion, but the slope is very susceptible to erosion, particu¬ 
larly near the water line. The soil loss from erosion will 
have the added impact of reducing the detention time in 
the wetland. This has not been a problem in clay-lined 
wetlands because the clay provides a good rooting me¬ 
dium. 

6.2.1.4 Liners 

Liners used for wetlands are the same as those typically 
used for lagoons and ponds. The materials include: 

Polyvinyl chloride (PVC) 

Polyethylene (PE) 

Polypropylene 

Most systems typically use 30 mil polyvinyl chloride 
(PVC) or high-density polyethylene (HDP). These may be 
prefabricated for small, individual-residence wetlands, but 



Grassed Berm 
(> 3:1 typical) 



Figure 6-1. Examples of constructed wetland berm construction 


112 














































they are usually constructed in place using conventional 
procedures for assembly, joint bonding, and anchoring. 
Liners also may include scrims, which are more costly. The 
scrim is a woven nylon or polypropylene net embedded in 
plastic or surrounding bentonite. Plastic liners with scrims 
are marketed under trade names such as Hypalon or XR- 
5. Several good resources are available for liner applica¬ 
tion and selection (EPA, 1993; EPA, 1994; Rumer and 
Mitchell, 1996). 

Liner punctures must be prevented during placement and 
subsequent construction activity. If the subgrade contains 
sharp stones, a geotextile fabric should be placed beneath 
the liner. A geotextile fabric or a layer of sand approximately 
5 cm (2 in) thick should be placed on top of the liner if 
crushed rock is used in a VSB system. The engineer should 
specify that the liner installer provides written approval of 
the condition of the subgrade as a condition prior to liner 
installation. 

Many membrane liners currently used require protec¬ 
tion from ultraviolet solar radiation. Conventional methods 
can be used to achieve protection, but VSB systems should 
not use a soil cover as UV protection since erosion may 
wash soil into the bed and result in local media clogging. 
Riprap material consisting of aggregate approximately 8- 
15 cm (3-6 in) in size is recommended for this application. 
This larger riprap will reduce the potential for weeds to 
become established and spread into the wetland. It can 
also withstand foot traffic for the life of the system. 

Clay liners also have been used. Manufactured liners 
using bentonite are common. The bentonite may be mixed 
with the native soils and compacted, or it may be in the 
form of pads or blankets consisting of bentonite between 
two scrims of finely woven polypropylene or polyethylene. 

Native soils may be used if they have sufficiently high 
clay content to achieve the necessary permeability. Usu¬ 
ally the state regulatory agency will specify the acceptable 
permeability. Typically, the clay liner must be 0.3 m (1 ft) or 
more in thickness to provide the necessary hydraulic bar¬ 
rier. In the case of a FWS, the surface of the clay layer 
should be well compacted to discourage root penetration 
by the emergent vegetation as the wetland matures. 

6.2.1.5 Inlet and Outlet Structures 

Inlet and outlet structures distribute the flow into the 
wetland, control the flow path through the wetland, and 
control the water depth. Multiple inlets and outlets spaced 
across either end of the wetland are essential to ensure 
uniform influent distribution into and flow through the wet¬ 
land. These structures help to prevent “dead zones” where 
exchange of water is poor, resulting in wastewater deten¬ 
tion times that can be much less than the theoretical de¬ 
tention times. 

In small- to medium-sized wetlands, perforated or slot¬ 
ted manifolds running the entire wetland width typically are 
used for both the inlets and outlets. Sizes of the mani¬ 
folds, orifice diameters, and spacing are a function of the 


projected flow rate. For example, the first cell of the FWS 
wetland in West Jackson County, Ml, is designed for an 
average flow of 2,270 m 3 /d (600,000 gpd). It uses a 300 
mm (12 in)-diameter PVC manifold for the inlet that ex¬ 
tends the full 76 m (250 ft) width of the cell. The manifold 
is perforated with 50 mm (2 in)-diameter orifices on 3 m 
(10 ft) centers. It rests on a concrete footing to ensure sta¬ 
bility and discharges to a 150 mm (6 in)-thick layer of coarse 
aggregate. A single inlet would not be suitable for a wide 
wetland cell such as this because it would not be possible 
to achieve uniform flow across the cell. Multiple weir boxes 
could be used as an alternative. Splitter boxes using “V” 
notched weirs or other methods can be used to divide the 
influent flow equally between the individual weir boxes. 
The weir boxes also can be used for measuring the influ¬ 
ent flow. Examples of these types of structures can be found 
in irrigation engineering textbooks. 

Where possible, the inlet manifold should be installed in 
an exposed position to allow access by the operator for 
flow adjustment and maintenance. Several alternatives to 
the simple drilled orifice can be used for flow distribution 
control. See Figure 6-2 for examples of inlet manifolds. 

In cold climates where extended periods of freezing 
weather are possible or where public exposure is an is¬ 
sue, a submerged inlet is necessary. In these instances, 
the simple perforated inlet manifold is used. Since it is not 
possible to adjust the level of the submerged manifolds 
after construction is completed, extra effort should be ex¬ 
pended to compact and grade the inlet and outlet zones to 
limit subsequent settling. It may be necessary to support 
the manifold on concrete footings where the underlying 
soils are potentially unstable. An accessible cleanout should 
be provided at each end of the submerged manifold to 
allow flushing if the manifold becomes clogged. Shut-off 
devices should be provided on all inlets to permit mainte¬ 
nance or resting of the wetland. 

In FWS wetlands, the encroachment of adjacent emer¬ 
gent vegetation may clog the manifold outlets with plant 
litter and detritus. This problem may be eliminated by con¬ 
structing a deep water zone approximately 1-1.3 m (3-4 
ft) deeper than the bottom of the rest of the wetland. The 
open area should be limited to 1 m (3 ft) in width. The 
manifold also can be enclosed in a berm of coarse riprap 
8-15 cm (3-6 in) in size. The coarse riprap inhibits plant 
growth. The open water design, however, allows easier 
access to the manifold for maintenance, but may encour¬ 
age wildlife visitation and the potential effluent quality deg¬ 
radation that accompanies it. 

Outlet structures help to control uniform flow through the 
wetland as well as the operating depth. If submerged out¬ 
let manifolds are used, they must be connected to a level 
control device that permits the operator to adjust the water 
depth in the wetland. This device can be an adjustable 
weir or gate, a series of stop logs, or a swiveling elbow 
(Figure 6-3). 

An alternative to submerged manifolds for inlet and out¬ 
let structures is multiple weir or drop boxes. These are 


1 


13 



Cleanout (both ends) 


3E 


®o o° 

oU 

fig 

°o°> 


otf 

o9o 

0 OoQ 


» 96 

rr fl ° 


0 

°lo 0 

•\° 

0 o 0 

o<?o 

do 0 
0 
0 


Control Valve 



a) Submerged Perforated Pipe 


Settled 

Sewage 



uPVC Pipe 


' • • * V.* 


;. * * ' Reed Bed 


♦ • ** ■ .- o . 

• • 0 . . • 


Wire Mesh Gabions 




Wire Mesh Gabion 
with 60 - 100mm Stones 


b) Gabion Feed 


Level Surface 


Soil Cover over Liner 


. * * * Reed Bed 


Back-filled 
with Stones 


• k ‘ 



90° Tees with "O" Ring Seals Inlet 


Wire Mesh Gabion (optional) 
with 60 - 100mm Stones 


c) Swivel Tee 


Figure 6-2. Examples of constructed wetland inlet designs 


114 















































































































Outlet 


Adjustable 

Weir 



Debris 

Screen 


Adjustable Weir 



Outlet 


Debris 

Screen 


a) Adjustable Weir 


Wire Mesh Gabion (optional) 
with 60 - 100mm Stones 


\ Wire Mesh Gabion (optional) 
with 60 - 100mm Stones 



Slotted Pipe 
Collector 


"O" Ring Joint 



c) 90° Elbow Arrangement 


Liner 


Slotted Pipe 
Collector 


b) Interchangeable Section 


Interchangeable Section 
of Pipe Fits "O" Ring Socket 


Figure 6-3. Outlet devices 


115 































































































































usually constructed of concrete, either cast in place or pre¬ 
fabricated. Several boxes must be installed across the width 
of the wetland to ensure uniform flow through the wetland. 
Preferred spacing varies from 5 to 10 m (15-30 ft) but may 
be as much as 15 m (50 ft) on center depending on the 
width of the wetland cell. Overflow rates should be limited 
to <200 m 3 /m-d (16,000 gpd/ft 2 ). Drop boxes do require a 
deep water zone immediately around them to minimize 
vegetation encroachment. In northern climates, the boxes 
are more susceptible to freezing than are submerged mani¬ 
folds. 

Debris screens may be placed in front of FWS wetland 
outlets. In Figure 6-3 there is an example of their place¬ 
ment in the outlet. The emergent vegetation in the wetland 
will drop many leaves, and storm events can uproot entire 
plants that float to the collection manifolds or outlet struc¬ 
tures. The screens will prevent the debris from clogging 
the downstream piping or treatment processes or impair¬ 
ing effluent quality. 

6.2.1.6 Media 

A soil or finer-rock medium is necessary as a matrix in 
both FWS wetlands and VSB systems for supporting emer¬ 
gent vegetation. In FWS wetlands, a layer of soil at least 
15 cm (6 in) deep is placed on the compacted bottom or 
liner to create the rooting medium for the intended emer¬ 
gent vegetation. This soil can be the topsoil removed dur¬ 
ing initial site preparation and grading or can be imported. 
Any loamy soil with acceptable agronomic properties is 
suitable. 

In VSB systems, the media provide the matrix for water 
flow as well as the planting medium. Gravel is the most 
commonly used media, but sand, crushed rock, and plas¬ 
tic media also have been used (Kadlec and Knight, 1996). 
Large gravel media are typically recommended to prevent 
clogging, whereas a smaller media layer can be used on 
top of a bed of larger gravel to provide a better rooting 
media (see Chapter 5). 

Most local sand and gravel suppliers have concrete ag¬ 
gregate available that can be screened for the coarse frac¬ 
tions to provide an acceptable media for VSB systems. 
The media should be washed to eliminate soil and other 
fines that can contribute to media clogging. Rounded river 
gravel is recommended over sharp-edged crushed rock 
because of the looser packing that the rounded rock pro¬ 
vides. Hard, durable stone (river gravel or crushed stone) 
is recommended. Crushed limestone, which is soft and 
easily disintegrates, should be avoided. 

6.2.2 Vegetation Establishment 

Vegetation and its litter is necessary for successful per¬ 
formance of FWS wetlands and contributes aesthetically 
to the appearance of both FWS wetlands and VSB sys¬ 
tems. Establishing this vegetation is probably the least fa¬ 
miliar aspect of wetland construction for most contractors. 
In recent years, a number of specialty firms have emerged 
with the expertise for selecting and planting the vegetation 


in these systems. Employing one of these firms is recom¬ 
mended for large projects if the construction contractor 
does not have prior wetland experience. 

6.2.2.1 Species Selection and Sources 

For wastewater treatment, macrophytes selected for 
planting should (1) be active vegetative colonizers with 
spreading rhizome systems, (2) have considerable biom¬ 
ass or stem densities to achieve maximum velocity gradi¬ 
ent and enhanced flocculation and sedimentation, and (3) 
be a combination of species that will provide coverage over 
the broadest range of water depths encountered (Allen et 
al., 1990). 

Wetland plants can be purchased from nurseries, col¬ 
lected in the wild, or grown for a specific project. No gen¬ 
eral recommendation can be made as to the best source 
of plants for a particular project. However, wild collected 
plants are usually the most desirable because they are 
more closely adapted to local environmental conditions, 
can be planted with limited storage, and offer a greater 
diversity. For large projects, commercial seedlings may be 
the most cost-effective alternative. The seedlings are sup¬ 
plied in suitable planting condition that allows use of me¬ 
chanical agricultural equipment for planting. 

6.2.2.2 Planting 

Establishing vegetation in a constructed wetland involves 
planting a suitable propagule at the appropriate time. Whole 
plants or dormant rhizomes and tubers are typically planted. 
Seeding has not been particularly successful because of 
stratification requirements of wetland seed and loss of seed 
from water action. 

In temperate climates, the prime planting period begins 
after dormancy has begun in the fall and ends after the 
first third of the summer growing season has passed. The 
planting period for herbaceous vegetation is broader than 
for woody plants. Early spring growing-season plantings 
have been the most successful (Allen et al., 1990). 

Planting seedlings or clumps is the simplest method. 
Some experience is necessary wittwhizomes to identify 
the node of the future shoot, which must be planted up¬ 
ward. Special anchoring may be necessary when the plant¬ 
ing medium is soft, plants are buoyant, or erosion will dis¬ 
turb the system. The soil should be maintained in a moist 
condition after planting. The water level can be increased 
slowly as new shoots develop and grow. The water level 
must never be higher than the tips of the green shoots or 
the plants will die. 

The macrophyte planting density can be as close as 0.3 
m (1 ft) centers or as much as 1 m (3 ft). The higher the 
density, the more rapid will be the development of a ma- I 
ture and completely functional wetland system. However, I 
high density plantings will increase construction costs sig¬ 
nificantly. If planted on 1 m (3 ft) centers, a wetland sys- i 
tern will take at least two full growing seasons to approach 
equilibrium and optimal plant-related performance objec- 


116 



tives. FWS wetlands should be planted more densely ow¬ 
ing to the role of the plants in the treatment process. How¬ 
ever, it may not be economical to plant very large FWS 
wetlands on 1 m (3 ft) centers if the total wetland area is 
intended to cover several thousand hectares. In such 
cases, plantings should be done in separate bands ex¬ 
tending the full width of the wetland cells to interrupt pref¬ 
erential flow of wastewater through the cells. VSB systems 
can be planted less densely. 

Water must be provided during the initial growth period. 
This can be complicated in large FWS wetlands because 
it may not be possible to plant the entire surface in a cell at 
one time. If mechanical equipment is used for planting, 
the unplanted areas should be kept dry until planting is 
complete. Since the bottom is sloped toward the discharge 
end, planting should start at the outlet and proceed toward 
the inlet. Sprinklers and shallow flooding have been used 
to keep the planted areas wet. If planting by hand, the whole 
area can be flooded with a few centimeters of water. The 
water depth can be increased gradually as the plant shoots 
grow until the design level is reached. If the FWS wetland 
is designed to treat a high-strength influent such as pri¬ 
mary treated wastewater, a cleaner water source or dilut¬ 
ing the wastewater with storm water or well water is rec¬ 
ommended for the initial planting and growth period so the 
plants are not overly stressed. If the intended influent is 
close to secondary quality, it can be used immediately. If 
an acceptable agronomic soil has not been used as the 
rooting media, a preliminary application of commercial fer¬ 
tilizers may be necessary. Use of fertilizers should be care¬ 
fully considered, however, because of the potential impacts 
of the nutrients that might escape in the effluent to the 
receiving water. 

In VSB systems, it is typical practice to flood the wetland 
cell to the surface of the media prior to planting and to 
maintain that level until significant growth has occurred. 
Later, the water level is lowered to the intended operating 
level. If the wetland is designed to treat septic tank or pri¬ 
mary effluent, clean water is recommended for the plant¬ 
ing phase. The high oxygen demand of the wastewater 
could inhibit initial plant growth. After a few weeks of plant 
growth, wastewater can be introduced. A layer of straw or 
hay mulch 15 to 20 cm (6-8 in) in thickness should be 
placed on the gravel surface to protect the new plants from 
the high summer surface temperatures that can occur on 
bare gravel surfaces. The mulch also is useful for provid¬ 
ing thermal insulation during the first winter of operation in 
northern climates. 

6.3 Start-up 

Start-up periods for FWS wetlands are necessary to 
establish the flora and fauna associated with the treatment 
processes. The start-up period will vary in length depend¬ 
ing on the type of design (FWS wetlands or VSB systems), 
the characteristics of the influent wastewater, and the sea¬ 
son of year. In FWS wetlands, the start-up period should 
1 be sufficient for the vegetation to become well established 
if the treatment objectives are to be met. The start-up pe¬ 


riod for VSB systems is less critical since its performance 
is less dependent on vegetation. 

6.3.1 Free Water Surface Wetlands 

FWS wetlands will not attain optimum performance lev¬ 
els until the vegetation and litter are fully developed and at 
equilibrium. The time required to reach this point is a func¬ 
tion of the planting density and season of the year. A wet¬ 
land with a high density planting that is started in the spring 
is likely to be fully developed by the end of the second 
growing season. A wetland with a low density planting 
started in late fall in a northern climate may require three 
years or more to achieve its intended treatment perfor¬ 
mance. 

Under ideal conditions, start-up of a FWS wetland should 
be delayed six weeks after planting to provide sufficient 
time for the emergent plants to acclimatize and grow above 
the working water level. When this is not possible, start-up 
must control the water level at less than plant height. How¬ 
ever, such rapid start-up will risk damage to the new plants 
and may prolong the time required for the system to reach 
optimum performance. 

When start-up is initiated, the water level must be gradu¬ 
ally raised to the design level by adjusting the flow control 
device at the outlet of each cell. This is done to allow the 
tops of the emerging vegetation to remain above water. If 
the influent is high strength, such as primary or septic tank 
effluent, it may be necessary to dilute the influent with clear 
water or recycle treated effluent to slowly increase the 
pollutant loadings to the wetland until the vegetation is 
acclimatized. 

During the start-up period, the operator should inspect 
the wetland several times per week. Plant health and 
growth should be observed, berms and dikes inspected 
for structural problems, water levels adjusted, and mos¬ 
quito emergence noted. In large areas of a FWS system 
where growth of the vegetation has failed, the macrophytes 
should be replanted to avoid the risk of short-circuiting of 
flow. The experience developed during this period will be 
helpful in determining the inspection frequency that will be 
required during the mature phase of the wetland. 

Treatment performance during the start-up period may 
not be representative of long-term expectations. Poorly 
established FWS wetlands with minimal vegetation will not 
perform acceptably. Influent TSS and associated pollut¬ 
ants will not be properly removed. Large open areas will 
permit algae blooms. The system will not perform differ¬ 
ently from a maturation pond, in that only pathogen kill will 
likely occur. Removal efficiency of TSS and its associated 
pollutants can be expected to improve as the plant canopy 
develops and increases in density. Removals of ammonia 
and phosphorus may be greater during the start-up period 
than after equilibrium is reached in new FWS wetlands, 
which have a new soil surface and rapidly growing veg¬ 
etation. Both conditions provide a rapid but short-term re¬ 
moval of these nutrients. Adsorption sites on soil particles 


117 










can take up both ammonia and phosphorus, and the nutri¬ 
ent uptake by the plants during the rapid growing phase 
can be significant. Within one or two years of start-up, how¬ 
ever, removal of phosphorus will decline. Removal of am¬ 
monia nitrogen will decline also unless the system is a 
FWS with substantial open areas. 

6.3.2 Vegetated Submerged Bed Systems 

Treatment in VSB systems is primarily BOD and TSS 
removal through the trapping of particulate material in the 
media. Some BOD removal may be reintroduced from bio¬ 
chemical methanogenisis of captured organic solids in the 
anaerobic environment. Biological denitrification also may 
occur if nitrates are present in the influent. Plants may take 
up nutrients in the wastewater, but this is usually not sig¬ 
nificant. Phosphorus removal during the start-up period will 
occur, but typically becomes minor as chemical exchange 
sites on the media are filled. Since the plants play an in¬ 
significant role in the treatment performance, equilibrium 
should be reached in less than one year unless high-ca¬ 
pacity media is employed. 

During the start-up period, the operator is primarily re¬ 
sponsible for adjusting the water level in the wetland. Typi¬ 
cally, the VSB systems will be filled to the surface of the 
media at the end of planting. As the plants begin to root, 
the water level can be gradually lowered to the design 
operating level. It may be necessary to add fertilizer dur¬ 
ing this period until sufficient nutrients are made available 
by the addition of wastewater. 

6.4 Operation and Maintenance 

Constructed wetlands are “natural” systems. As a result, 
operation is mostly passive and requires little operator in¬ 
tervention. Operation involves simple procedures similar 
to the requirements for operation of a facultative lagoon. 
The operator must be observant, take appropriate actions 
when problems develop, and conduct required monitoring 
and operational monitoring as necessary. The most criti¬ 
cal items in which operator intervention is necessary 
areAdjustment of water levels 

• Maintenance of flow uniformity (inlet and outlet struc¬ 
tures) 

• Management of vegetation 

• Odor control 

• Control of nuisance pests and insects 

• Maintenance of berms and dikes 

6.4.1 Water Level and Flow Control 

Water level and flow control are usually the only opera¬ 
tional variables that have a significant impact on a well- 
designed constructed wetland’s performance. Changes in 
water levels affect the hydraulic residence time, atmo¬ 
spheric oxygen diffusion into the water phase, and plant 
cover. Significant changes in water levels should be in¬ 


vestigated immediately, as they may be due to leaks, 
clogged outlets, breached berms, storm water drainage, 
or other causes. 

Seasonal water level adjustment helps to prevent freez¬ 
ing in the winter. In cold climates, the water levels should 
be raised approximately 50 cm (18 in) in late fall until an 
ice sheet develops. Once the water surface is completely 
frozen, the water levels can be lowered to create an insu¬ 
lating air pocket under the ice and snow cover to maintain 
higher water temperatures in the wetland (Kadlec and 
Knight, 1996; Grits and Knight, 1990). This procedure is 
used for both FWS wetlands and VSB systems. 

6.4.2 Maintenance of Flow Uniformity 

Maintaining uniform flow across the wetland through in¬ 
let and outlet adjustments is extremely important to achieve 
the expected treatment performance. The inlet and outlet 
manifolds should be inspected routinely and regularly ad¬ 
justed and cleaned of debris that may clog the inlets and 
outlets. Debris removal and removal of bacterial slimes 
from weir and screen surfaces will be necessary. Sub¬ 
merged inlet and outlet manifolds should be flushed peri¬ 
odically. Additional cleaning with a high-pressure water 
spray or by mechanical means also may become neces¬ 
sary. 

Influent suspended solids will accumulate near the in¬ 
lets to the wetland. These accumulations can decrease 
hydraulic detention times. Over time, accumulation of these 
solids will require removal. VSB systems cannot be 
desludged easily without draining and removing the me¬ 
dia. Therefore, VSB systems should not be considered for 
treating wastewaters with high suspended-solids loads, 
such as facultative pond effluents, which have high algal 
concentrations. 

6.4.3 Vegetation Management 

Routine maintenance of the wetland vegetation is not 
required for systems operating within their design param¬ 
eters and with precise bottom-depth control of vegetation. 
Wetland plant communities are self-maintaining and will 
grow, die, and regrow each year. Plants will naturally spread 
to unvegetated areas with suitable environments (e.g., 
depth within plant’s range) and be displaced from areas 
that are environmentally stressful. Operators must control 
spreading into open water areas that are intended by de¬ 
sign to be aerobic zones through harvesting. 

The primary objective in vegetation management is to 
maintain the desired plant communities where they are 
intended to be within the wetland. This is achieved through 
consistent pretreatment process operation, small, infre¬ 
quent changes in the water levels, and harvesting plants 
when and where necessary. Where plant cover is deficient, 
management activities to improve cover may include wa¬ 
ter level adjustment, reduced loadings, pesticide applica¬ 
tion, and replanting. 

Harvesting and litter removal may be necessary depend¬ 
ing on the design of the system. Plant removal from some 


118 






FWS wetlands may be required to meet the treatment 
goals, but a well-designed and well-operated VSB system 
should not require routine harvesting. Harvesting of 
Phragmites at the height of the growing season and just 
before the end of the growing season does help to remove 
some nitrogen from the system, but phosphorus removal 
is limited (Suzuki et al., 1985). Winter burning of vegeta¬ 
tion can be used to control pests. 

6.4.4 Odor Control 

Odors are seldom a nuisance problem in properly loaded 
wetlands. Odorous compounds emitted from open water 
areas are typically associated with anaerobic conditions, 
which can be created by excessive BOD and ammonia 
loadings. Therefore, reducing the organic and nitrogen 
loadings can control odors. Alternatively, aerobic open 
water zones interspersed in areas between fully vegetated 
zones introduce oxygen to the system. Turbulent flow struc¬ 
tures such as cascading outfall structures and channels 
with hydraulic jumps, which are employed to introduce 
oxygen into the system effluent, can generate serious odor 
problems through stripping of volatile compounds such as 
hydrogen sulfide, if the constructed wetland has failed to 
remove these constituents. 

6.4.5 Control of Nuisance Pests and 
Insects 

Potential nuisances and vectors that may occur in FWS 
wetlands include burrowing animals, dangerous reptiles, 
mosquitoes, and odors. An infestation of burrowing ani¬ 
mals such as muskrats and nutria can seriously damage 
vegetation in a system. These animals use both cattails 
and bulrushes as food and nesting materials. These ani¬ 
mals can be controlled during the design phase by de¬ 
creasing the slope on berms to 5:1 or using a coarse riprap. 
Temporarily raising the operating water level may also dis¬ 
courage the animals. Live trapping and release may be 
successful, but in most cases it has been necessary to 
eliminate the animals. Fencing has had little success. 

Dangerous reptiles are common in the southeastern 
states. The most common are snakes, particularly the water 
moccasin, and alligators. It is difficult to control these ani¬ 
mals directly. Warning signs, fencing, raised boardwalks, 
and mowed hiking trails can be used to minimize human 
contact with the animals. Operators should be made aware 
of the dangers and preventive actions that can be taken to 
avoid dangerous situations. 

Mosquito control is a critical issue in FWS wetlands. In 
warm climates, wetlands have been seeded with mosquito 
fish (Gambusia) and dragonfly larvae to control mosqui¬ 
toes. Mosquito fish also can be used in northern climates, 
but they need to be restocked each year. However, mos¬ 
quito fish have difficulty reaching all parts of the wetland 
when the accumulation of litter is too dense, particularly if 
cattails are grown. Other natural control methods have in¬ 
cluded erecting bat and bird houses. Desirable birds in¬ 
clude purple martins and swallows. Bacterial larvicides, 
BTI (Bacillus thuringiensis israelensis), and BS Bacillus 


sphaericus) have been used successfully in a number of 
wetlands. 

6.4.6 Maintenance of Berms and Dikes 

Berms and dikes require mowing, erosion control, and 
prevention of animal burrows and tree growth. When the 
wetland is operated at a shallow depth, periodic removal 
of tree seedlings from the wetland bed may be necessary. 
If the trees are allowed to reach maturity, they may shade 
out the emergent vegetation and with it the necessary con¬ 
ditions to enhance flocculation, sedimentation, and deni¬ 
trification. 

6.5 Monitoring 

Routine monitoring is essential in managing a wetland 
system. In addition to regulatory requirements, inflow and 
outflow rates, water quality, water levels, and indicators of 
biological conditions should be regularly monitored and 
evaluated. Monitoring of biological conditions includes 
measurement of microbial populations and monitoring 
changes in water quality, percent cover of dominant plant 
species, and benthic macroinvertebrate and fish popula¬ 
tions at representative stations. Overtime, these data help 
the operator to predict potential problems and select ap¬ 
propriate corrective actions. 

6.6 References 

Allen, H.H., G.J. Pierce, and R. Van Wormer. 1990. Consider¬ 
ations and techniques for vegetation establishment in con¬ 
structed wetlands. In: D.A. Hammer (ed.) Constructed wet¬ 
lands for wastewater treatment, municipal, industrial and 
agricultural. Chelsea, Ml: Lewis Publishers, Inc. 

Grits, M.A. and R.L. Knight. 1990. Operations optimization. In: 
D.A. Hammer (ed.) Constructed wetlands for wastewater 
treatment, municipal, industrial and agricultural. Chelsea, 
Ml: Lewis Publishers, Inc. 

Kadlec, R.H. and R.L. Knight. 1996. Treatment wetlands. Boca 
Raton, FL: CRC Press LLC. 

Rumer and Mitchell, 1996. Assessment of barrier containment 
technologies. NTIS report # PB 96-180583. 

Suzuki, T., A.G. Wathugala, and Y. Kurihara. 1985. Preliminary 
studies on making use of Phragmites australis for the re¬ 
moval of nitrogen, phosphorus, and COD from the waste 
water. UNESCO’s Man and Biosphere Programme in Ja¬ 
pan, Coordinating Committee on MAB Programme, pp. 95- 
99. 

U.S. Environmental Protection Agency (EPA). 1993. Report of 
workshop on geosynthetic clay liners. EPA/600/R-93/171. 
Office of Research and Development, Washington, DC. 

U.S. Environmental Protection Agency (EPA). 1994. Seminar 
Publication: Design, operation, and closure of municipal 
solid waste landfills. EPA/625/R-94/008. Office of Re¬ 
search and Development, Cincinnati, OH. 


119 






Chapter 7 

Capital and Recurring Costs of Constructed Wetlands 


7.1 Introduction 

The major items included in capital costs of constructed 
wetlands are 

• Land costs 

• Site investigation 

• Clearing and grubbing 

• Excavation and earthwork 

• Liner 

• Media 

• Plants 

• Inlet structures 

• Outlet structures 

• Fencing 

• Miscellaneous piping, pumps, etc. 

• Engineering, legal, and contingencies 

• Contractor’s overhead and profit 

Most of these costs are directly dependent on the de¬ 
sign treatment area of the system, and the unit costs for 
almost all are essentially the same for FWS and VSB sys¬ 
tems. The major difference between the two concepts is 
the media cost (Table 7-1). In the case of a VSB, a 60 cm 
(2 ft) depth of gravel typically fills the bed, whereas the 
medium for a FWS wetland consists of 15 cm (6 in) or 
more of topsoil used as growth media for the wetland veg¬ 
etation. 

7.2 Construction Costs 

7.2.1 Total Construction Costs 

The cost data in this report were obtained from site vis¬ 
its to nine operational constructed wetland systems and 
from related published sources. The nine systems were 
Areata, CA; Gustine, CA; Mesquite, NV; Ouray, CO; West 
Jackson County, MS; Mandeville, LA; Sorrento, LA; 


Carville, LA; and Ten Stones, VT. This group includes four 
FWS wetlands and five VSBs with design flows ranging 
from 0.3 to 175 L/s (6,700 gpd to 4 mgd). The start-up 
dates for these subsystems range from 1986 (Areata, CA) 
to 1997 (Ten Stones, VT). In order to provide a common 
base for comparison, all costs have been adjusted with 
the appropriate Engineering News Record (ENR) Construc¬ 
tion Cost Index (CCI) factor to August 1997 (ENR CCI = 
5854). 

Unfortunately, it is not possible to extract the individual 
line-item construction costs listed in Table 7-1 for most 
existing wetland systems because their construction con¬ 
tracts were let as lump sum bids for entire projects. In many 
cases, the situation is further confounded since the lump 
sum bids also may include preliminary treatment compo¬ 
nents, pumping stations, and community collection sys¬ 
tems. In addition, local conditions and site characteristics 
also significantly affect wetland system costs. VSB wet¬ 
lands in southern Louisiana, for example, pay a high price 
for imported gravel since none is available locally. Some 
existing wetland systems were converted from existing la¬ 
goon cells. In these cases, the costs for clearing and grub¬ 
bing and excavation and earthwork would be minimal. As 
a result of these factors, it is not possible to derive general 
nationally applicable cost-per-hectare unit cost. The best 
that can be achieved is a range of costs that may be use¬ 
ful for an order-of-magnitude preliminary estimate. 

Table 7-2 presents a summary of technical and cost data 
for the nine constructed wetland systems included in the 
EPA case study visitations. The costs listed in the table 
are the estimated construction costs for the wetland com¬ 
ponent in each system at the time the system was con¬ 
structed. It is difficult to draw general conclusions from the 
data because of the many variables involved. The sys¬ 
tems listed were designed with different design models and 
procedures to achieve different water quality goals, so the 
relationship between the treatment areas provided and the 
design flow rates is not meaningful. The only nearly con¬ 
sistent factor is that land costs were zero in all cases ex¬ 
cept Ouray, CO, because the land was already in the pos¬ 
session of the system owner. The costs given are based 
on actual construction costs and do not include a factor for 
system design or site investigation. In most cases, the site 
investigation costs for these nine systems were minimal 


120 






Table 7-1. Cost Comparison of 4,645 m 2 Free Water Surface Constructed Wetland and Vegetated Submerged Bed 


Free Water Vegetated 

Surface Wetland Submerged Bed 


Item 

Units 

Unit Price 

Total Cost ($) 

% of Total 

Total Cost ($) 

% of Total 

Excavation/ 

Compaction 

m 3 

$2.30 

13,000 

19.4 

13,000 

10.7 

Soil (45 cm) 

m 3 

$1.30 

2,800 

4.2 

na 

— 

Gravel (60 cm) 

m 3 

$20.95 

Na 

— 

51,900 

42.6 

Liner (30 mil PC) 

m 2 

$3.75 

19,250 

28.7 

19,250 

15.8 

Plants 

Each 

$0.60 

7,500 
(60 cm o.c.) 

11.2 

13,330 
(45 cm o.c.) 

10.9 

Plumbing 

Lump sum 


7,500 

11.2 

7,500 

6.1 

Control Structures 

Lump sum 


7,000 

10.4 

7,000 

5.7 

Other 

Lump sum 


10,000 

67,050 

14.9 

100.0 

10,000 

121,980 

8.2 

100.0 


* 4,645 m 2 = 50,000 ft 2 


Table 7-2. Technical and Cost Data for Wetland Systems Included in 1997 Case Study Visitations 


Location 

Startup 

Date 

Area 

(hectares) 

Flow 

(m 3 /s) 

No. 

Cells 

Liner' 

Berm 

Const 2 

Land 

Cost 

Const. 

Cost 

$/m 2 

Adj. Cost 3 
$/m 2 

Free Water Surface Wetlands 









Areata, CA 

1986 

3.0 

66.2 

3 

No 

No 

$0 

$225,000 

7.43 

10.12 

Gustine, CA 

1987 

9.8 

22.8 

24 

No 

Yes 

$0 

$882,000 

9.04 

12.27 

Ouray, CO 

1993 

0.9 

8.7 

4 

Yes 

Yes 

$55,000 

$108,500 

12.16 

13.02 

W.J.C. MS 

1997 

20.2 

54.8 

7 

No 

Yes 

$0 

$700,000 

3.44 

3.44 

Vegetated Submerged Beds 









Carville, LA 

1986 

0.3 

3.4 

1 

No 

Yes 

$0 

$100,000 

38.64 

52.64 

Mandeville.LA 

1990 

2.3 

34.2 

3 

No 

No 

$0 

$590,000 

26.05 

32.18 

Mesquite, NV 

1991 

1.9 

9.1 

12 

No 

No 

$0 

$515,000 

27.13 

32.83 

Sorrento, LA 

1991 

0.07 

1.14 

1 

No 

Yes 

$0 

$75,000 

103.44 

125.18 

Ten Stones, VT 

1997 

0.04 

0.16 

2 

Yes 

Yes 

$0 

$40,000 

84.66 

89.66 


’A ‘No’ response is compacted native soil or pre-existing pavement 
2 A ‘No’ response had pre-existing lagoon berms 
Adjusted to August, 1997 costs (ENR CCI = 5854) 

Hectares = 2.47 acres; m 3 /s = 22.83 mgd; m 2 = 10.76 ft 2 


since information pertaining to soil characteristics and ground 
water conditions was already available. 

There is some evidence of economy of scale in the tabulated 
data; the 20.2-hectare (50-acre) FWS wetland expansion at West 
Jackson County, MS, is the largest system listed and had the 
lowest unit cost. Contributing factors are believed to be the lack 
of a liner and minimal berm construction because of the small 
number of relatively large cells selected for this project (seven 
cells). Ouray, CO, had the highest unit costs listed for FWS wet¬ 
lands. This system required the use of a membrane liner and 
had significant construction costs for berms because of the num¬ 
ber of small cells, in this case four cells on 0.89 hectares (2.2 
acres). The system at Gustine, CA, shows a higher unit cost 
than Areata, CA, primarily due to the extra berm construction 
involved. Other sources indicate that the cost per hectare for 


large FWS systems is about one-third the cost of smaller FWS 
wetlands, similar to the range presented in Table 7-2. The con¬ 
struction costs for FWS wetlands might therefore range from 
about $34,600 per hectare to $237,200 per hectare ($14,000/ 
acre to $96,000/acre in 1997$), depending on the size of the 
system, the number of cells and berms, and the need for a mem¬ 
brane liner. The size of the system will depend on the water- 
quality goals and local climatic conditions. Additional costs will 
include any site investigation and engineering design, pre- and/ 
or post-treatment components, means for transferring waste- 
water to the treatment site and effluent from the site, and land 
costs. 

Economy of scale is apparent for the five VSB systems listed 
in Table 7-2. The average unit cost for the two smallest sys¬ 
tems is at least twice that of the three larger systems. An- 


121 











other major factor in these cost differences is due to the 
significant differences in the local costs of the rock and 
gravel media used. Sorrento, LA, for example, used 
crushed limestone imported from Mexico because speci¬ 
fied aggregate was not available in that part of Louisiana. 
In comparison, Mesquite, NV, used available media from 
a nearby gravel pit. Local gravel also was available for the 
Ten Stones, VT, system, but the system is conservatively 
designed for the cold winter climate and has a higher unit 
cost than Mesquite, NV. If the Sorrento costs are omitted, 
the construction costs for VSB wetlands may range from 
$321,200 per hectare to $897,000 per hectare ($130,000/ 
acre to $363,000/acre in 1997$), depending on the size of 
the system, the need for a liner, and the local costs of gravel. 
The size of the system will depend primarily on the water- 
quality goals and to a lesser degree on local climatic con¬ 
ditions. Additional costs will include any site investigation 
and engineering design, pre- and/or post-treatment com¬ 
ponents, means for transferring wastewater to and/or from 
the treatment site, and land costs. These costs for VSB 
wetlands are significantly higher than the highest costs cited 
for FWS wetlands. 

7.2.2 Geotechnical Investigations 

Only four of the systems listed in Table 7-2 did not utilize 
any preliminary geotechnical investigations. Three of these, 
Areata, CA; Gustine, CA; and Mesquite, NV, utilized exist¬ 
ing lagoon cells, and the underlying soil conditions were 
presumably already known. The fourth system, Ten Stones, 
VT, was relatively small, and the state regulatory agency 
required a membrane liner regardless of the underlying 
soil characteristics, so a geotechnical investigation was 
not considered necessary. The other five systems utilized 
some shallow borings to verify expected soil conditions at 


the wetland site. The only system that retained cost data 
for this activity was Mandeville, LA, where approximately 
$15,000 was allocated in 1989 for site surveys and soil 
borings in the wetland area. The updated cost for survey¬ 
ing and soil borings at the Mandeville wetland site would 
be about $2,720 per hectare ($1,100/acre) in 1997$. 

7.2.3 Clearing and Grubbing 

Three of the systems listed in Table 7-2 required clear¬ 
ing and grubbing as part of their site preparation. Techni¬ 
cal details and related costs are listed in Table 7-3. The 
cost data in Table 7-3 are compatible with experience in 
the general construction industry. Costs for clearing and 
grubbing on relatively level land can range from $4,940 
per hectare ($2,000/acre) for brush and some small trees 
to $12,355 per hectare ($5,000/acre) for a tree-covered 
site. Campbell and Ogden (1999) indicate southwestern 
costs at $2,965 per hectare ($1,200/acre). 

7.2.4 Excavation and Earthwork 

Excavation and earthwork typically includes grading the 
wetland site to finished grade, constructing berms and 
access ramps, and in the case of FWS wetlands, reserv¬ 
ing and replacing topsoil in the bed to serve as the vegeta¬ 
tion growth medium. Table 7-4 summarizes available cost 
data from the 1997 survey. 

An economy of scale is expected for earthwork costs 
and is evident in the data for the small Ten Stones, VT, 
project where excavation costs were three times greater 
than the larger municipal-sized systems. All three sites 
listed were on relatively level land, with soils ranging from 
clay at West Jackson County, MS, to silty loam at Ten 
Stones, VT. The average cost for the two municipal sys- 


Table 7-3. Clearing and Grubbing Costs for EPA Survey Sites 


Location 

Site Area 
(hectares) 

Total 

Cost 

Cost / 

Hectare 

Adj. Cost 1 
($/hectare) 

West Jackson Co., MS 2 

20.2 

$100,000 

$4940 

4940 

Sorrento, LA 3 

1.6 

$7,000 

$4325 

5235 

Ouray, CO 4 

2.0 

$22,700 

$11,120 

12,050 

'Adjusted to August, 1997 $ (ENR CCI = 5854 

2 Ground cover: brush and sparse tree seedlings 

3 Ground cover: brush 

4 Ground cover: trees 

Hectares = 2.47 acres 





Table 7-4. Excavation and Earthwork Costs for EPA Survey Sites 

Site Area Quantity Total 

Location (hectares) (m 3 ) Cost 

$/Hectare 

$/m 3 

Adj. Cost' 
($/m 3 ) 

W. Jackson Co., MS 

20.2 

52,000 

$408,000 

20,165 

7.85 

8.29 

Gustine, CA 

9.7 

34,400 

$200,000 

20,600 

5.81 

8.03 

Ten Stones, VT 

0.04 

355 

$8,200 

184,200 

23.16 

23.16 


'Adjusted to August, 1997 $ (ENR CCI = 5854 
Hectares = 2.47 acres 
m 3 = 1.31 yd 3 


122 





















terns shown is about $8.17 per cubic meter ($6.25/yd 3 ). 
About one-third of that cost could be assigned to excava¬ 
tion of the wetland bed to grade (on relatively level land), 
with the remainder for berm and ramp construction and 
reservation and replacement of topsoil for the FWS sys¬ 
tem. Campbell and Ogden (1999) suggest $1.96 to $3.27 
per cubic meter ($1.50 to $2.50/yd 3 ) as a default value for 
preliminary estimates. 

7.2.5 Liner Costs 

A variety of materials, including the in situ native soils, 
have been used as liner material depending on the re¬ 
quirements of the regulatory agencies. The majority of the 
systems listed in Table 7-2 utilized the existing on-site soils 
for their liner material. Two of the remaining systems listed 
in Table 7-2 used plastic membrane liners. Table 7-5 sum¬ 
marizes these costs. These costs reflect the economy of 
scale available for larger systems. The unit cost at Ouray 
was $5.27 per m 2 ($0.49/ft 2 ), while $10.01 per m 2 ($0.93/ 
ft 2 ) was found at Ten Stones (1997$). Ouray used a 30-mil 
HDPE liner, and Ten Stones used a prefabricated 30-mil 
PVC liner. Other liner materials are also available, and typi¬ 
cal large system costs for some of these are presented in 
Table 7-6. Where soils are rocky, a geotextile fabric or layer 
of sand may be necessary beneath the synthetic liner to 
protect it from punctures. The liner will add $0.54 to $0.86/ 
m 2 ($0.05 to $0.08/ft 2 ) to the costs presented in Table 7-6. 
The costs of compaction and testing of clay liners can ex¬ 
ceed $3.23/m 2 ($0.30/ft 2 ). 


Table 7-5. Liner Costs for EPA Survey Sites 


Location 

Treatment Area 
(hectares) 

Total Cost/ 

Cost Hectare 1 

Adj. Cost 2 
($/hectare) 

Ouray, CO 

Ten Stones, VT 

1.36 3 

0.045 

$64,000 $46,930 

$4,500 $100,175 

52,725 

100,175 

'Represents cost per hectare of treatment area. Actual liner area is 
larger to cover berms, etc. 

2 Adjusted to August, 1997 $ (ENR CCI = 5854) 

3 Lined area at Ouray, CO includes lagoons and wetland cells. 

Hectares = 2.47 acres = 10,000 m 2 


Table 7-6. Typical Installed Liner Costs for 9,300 Square Meter 
(100,000 ft 2 ) Minimum Area 

Liner Material $/m 2 

Bentonite (9.8 kg/m 2 and harrowed in place) 

0.52-0.60 

Clay impregnated geosynthetic 

0.37-0.60 

Asphalt concrete 

0.60-0.75 

Butyl rubber (50 mm thickness) 

0.60 

PVC (30 mil) 

0.28-0.40 

HDPE (40 mil) 

0.35-0.40 

Hypalon (30 mil 

0.55 

Hypalon (60 mil) 

0.60-0.70 

PPE (30 mil) 

0.58-0.60 

Reinforced PPE (30 mil) 

0.65 

XR-5 

0.85-0.92 


m 2 = 10.76 ft 2 


7.2.6 Media Costs 

The media in a FWS wetland are the soils placed on top 
of the prepared bottom of the bed which serve as the growth 
medium for the emergent vegetation in the system. A simi¬ 
lar layer of topsoil is also usually applied to the berm slopes 
to allow their revegetation. Placement of these soil layers 
is usually included in the earthwork costs previously dis¬ 
cussed. 

The media used in a VSB are the gravel or rock placed 
in the bed. They serve to support the growth of the vegeta¬ 
tion and to provide physical filtration, flocculation, sedimen¬ 
tation, and surfaces for attached microbial growth and ad¬ 
sorption to occur. Several different sizes of rock and gravel 
can be used in these systems. At the sites visited in the 
EPA study, medium-sized gravel, 20-25 mm in diameter 
(0.75-1 in), was used for treatment. Coarser rock, 40-50 
mm in diameter (1.5-2 in), was used to surround the inlet 
and outlet manifolds, and a layer of pea gravel, 5-10 mm 
in diameter (1/4-3/8 in), was sometimes used to cap the 
gravel in the treatment bed. Coarse stone, 10-15 cm in 
diameter (4-6 in), is sometimes used to cover the exposed 
liner on the side slopes and to reduce the risk of burrowing 
animals. The unit cost of these materials depends on the 
size of the material, the volume needed, and the distance 
from the source to the wetland site. The media is usually 
the most expensive part of the construction of a VSB, po¬ 
tentially representing 40 to 55% of total construction costs 
(Table 7-1). Table 7-7 summarizes the costs for these ma¬ 
terials as derived from the 1997 site visitations. 

The local availability of gravel and the transport distance 
to the VSB site are the key factors influencing media costs. 
As a result, the cost data in Table 7-7 should only be used 
for preliminary estimates. Local sources should be con¬ 
tacted for a detailed construction cost estimate. Based on 
the data in Table 7-7, the media costs (for the main bed) 
range from $74,130 to $133,440 per hectare ($30,000 to 
$54,000/acre) depending on the local availability of suit¬ 
able material. 

7.2.7 Plants and Planting Costs 

Plant materials sometimes can be obtained locally by 
cleaning drainage ditches. It is also possible to develop an 
on-site nursery at the wetland construction site if there is 
sufficient advance time, or grow plant sprigs or seedlings 
from seed and transplant these to the wetland cells. A large 
and expanding number of commercial nurseries also exist 
and can supply a large variety of plant species for these 
wetlands systems. The majority of the systems listed in 
Table 7-2 were planted with commercial nursery stock. 
Small systems are typically planted by hand; large sys¬ 
tems can use mechanical planters, and nursery-grown 
sprigs or plant seedlings are advantageous for the pur¬ 
pose. Hydroseeding has been successful with Typha seeds 
(Gearheart et al., 1998). Table 7-8 summarizes available 
cost data for plants and planting from the 1997 survey sites. 
Campbell and Ogden (1999) quote a range of $0.50 to 
$1.00 per plant in place as a default value, while Gearheart 
et al. (1998) estimate $12,355 per hectare ($5,000/acre) 
for a total planting cost. 


123 
















Table 7-7. Media Costs for VSBs from EPA Survey Sites 



Media Size 

Media Depth 

Media Quantity 

Cost 

Cost 

Adj. Cost 1 

Location 

(mm) 

(m) 

(m 3 /hectare) 

($/m 3 ) 

(S/hectare) 

($/hectare) 

Mesquite, NV 

Bed: 10-25 

0.8 

8,140 

10.99 

89,380 

108,230 

Carville, LA 

Top: 20 

0.15 

1,525 

27.14 

41,325 

44,735 

Bed: 40-75 

0.60 

6,100 

20.21 

123,160 

133,320 

Ten Stones, VT 

Top: 10 

0.15 

1,523 

25.07 

38,180 

38,180 


Bed: 20-25 

0.60 

6,095 

12.01 

73,180 

73,180 


Outlets: 50 

0.60 

725 

7.85 

5,680 

5,680 


RipRap: 100 

0.12 

445 

18.31 

8,130 

8,130 

'Adjusted to August, 1997 $ (ENR CCI = 5854) 





m 3 /hectare = 1.89 yd 3 /acre 






Table 7-8. Costs for Wetland Vegetation and Planting from EPA Survey Sites 






Plant 




Adj. Planting 


Plant 

Density 

Planting 

Plant Cost 

Planting Cost 

Cost' 

Location 

Type 

(no./hectare) 

Method 

($/plant) 

($/hectare) 

($/hectare) 

Ten Stones, VT 

Cattails 

35,830 

Hand 

0.23 

Not available 

Not available 


Bulrush 

35,830 

Hand 

0.23 

Not available 

Not available 

Mandeville, LA 

Bulrush 

15,000 

Hand 

Not available 

2,965 

3,670 

Carville, LA 

Pickerel Weed 

34,595 

Hand 

Local sources 

2,470 

3,370 


Arrowhead 

34,595 

Hand 

Local sources 

2,470 

3,370 

Sorrento, LA 

None 

None 

None 

None 

None 

None 

Gustine, CA 

Bulrush 

46,950 

Mechanical 

Not available 

3,460 

4,595 


Cattails 

11,860 

Mechanical 

Not available 

1,975 

2,695 

Ouray, CO 

Cattails 

13,465 

Hand 

Local sources 

Not available 

Not available 

Bulrush 

13,465 

Hand 

Local sources 

Not available 

Not available 

West Jackson Co., MS 

Cattails 

11,860 

Mechanical 

0.38 

4,450 

4,450 

Mesquite, NV 

Bulrush 

Not available 

Hydroseeding 

Not available 

Not available 

Not available 

Areata, CA 

Bulrush 

9,885 

Hand 

Not available 

Not available 

Not available 

’Adjusted to August, 1997 costs (ENR CCI 

= 5854) 






Table 7-9. Costs for Inlet and Outlet Structures from EPA Survey Sites 

Location Structure Type Weir Type 

Cost 

($/structure) 

Adj. Cost 1 
($/structure) 

West Jackson Co., MS 

Concrete box 

Bolted plate 

2,500 

2,500 

Carville, LA 

Concrete box 

Shear Gate 

3,900 

4,400 

Gustine, CA 

Concrete box 

Stop log 

1,125 

1,500 

Ten Stones,VT 

100 mm PVC manifold 

None 

1,500 

1,500 


'Adjusted to August, 1997 $ (ENR CCI = 5854) 


7.2.8 Cost of Inlet and Outlet Structures 

The inlet and outlet structures for most small- to moder¬ 
ate-sized wetland systems are typically some variation of 
a perforated manifold pipe. Large wetland systems typi¬ 
cally use multiple drop or weir boxes for both inlets and 
outlets. Adjustable water level outlet structures should be 
used to control the water level in the wetland cell. If the 
outlet is a pipe manifold, a water level-control structure 


must be added, which should cost about the same as a 
weir box. Table 7-9 summarizes the available cost data 
from the 1997 survey. 

7.2.9 Piping, Equipment, and Fencing 
Costs 

These items include the piping to transfer the wastewa¬ 
ter to the wetland, the piping from the wetland to a dis- 


124 


















charge point, and any pumps required for either of those 
purposes. Fencing is typically installed around all munici¬ 
pal wastewater treatment systems, but has not usually been 
required around the smaller VSB wetland beds due to the 
low risk of public contact and exposure to the wastewater. 
None of these features are unique to wetland systems, 
and costs for these items were not available at the sites 
included in the 1997 EPA survey. 

7.2.10 Miscellaneous Costs 

These costs include engineering design and legal fees, 
construction contingencies, and profit and overhead for the 
construction contractor. These costs are not unique to 
wetland systems and are usually expressed as a percent¬ 
age of the total construction costs when preparing an esti¬ 
mate. Mobilization and bonding are also typically included 
in the construction costs. Typical values for miscellaneous 
costs are as follow: 

• Mobilization, 5% of direct costs 

• Bonds, 3% of direct costs 

• Engineering design services, 15% of capital costs 

• Construction services and start-up, 10% of capital costs 

• Contractor’s overhead and profit, 15% of capital costs 

• Contingencies, 15% of capital costs 

The following example illustrates the application of these 
factors. Assume direct project construction costs (i.e., la¬ 
bor, materials, equipment, etc.) are $300,000, therefore: 


Direct construction costs 

$300,000 

Mobilization, 5% 

$15,000 

Bonds, 3% 

$9,000 

Capital cost of construction 

$324,000 

Engineering, 15% 

$48,600 

Start-up, 10% 

$32,400 

Overhead, profit and contingencies, 30% 

$97,200 

Total capital costs 

$502,200 


7.2.11 Construction Cost Summary 

The major cost factors for both VSB and FWS wetlands 
are compared in Table 7-10. The tabulated data are drawn 
from previous tables for an assumed 0.405 hectare (one 
acre) wetland with a membrane liner. The cost data are in 

[ terms of dollars per hectare, and the percentage data are 
percent of total cost. The latter can be used to determine 
which system components are likely to be the most ex¬ 
pensive. The cost data shown do not include the costs of 
the land, mobilization, fencing, landscaping, pre- or post¬ 
treatment units, or the transfer piping to and from the wet¬ 
land site, and should only be used for preliminary, order- 
of-magnitude cost estimates. 


The cost of gravel media for the VSB is the most expen¬ 
sive item in Table 7-10, followed by the membrane liner for 
both types of wetlands. The cost of the gravel media in the 
VSB controls the cost regardless of the type of liner used. 
If site conditions allow for the compaction of native clay 
soil to produce an acceptable ground water barrier in lieu 
of a synthetic membrane liner, then the liner costs can be 
eliminated. In this case, perforated pipe manifolds for inlet 
and outlet structures are used instead of concrete weir 
boxes. 

7.3 Operation and Maintenance Costs 

The operation and maintenance of constructed wetland 
systems designed for wastewater treatment are relatively 
simple and require minimal time. They are similar to, but 
somewhat more than, the O&M requirements for a facul¬ 
tative pond. Most of the operator’s time at a wetland treat¬ 
ment system is spent servicing pumps, headworks, disin¬ 
fection, and other conventional components in the process. 
Animal (i.e., nutria, muskrats) control, vector (mosquitoes) 
control, and NPDES monitoring are probably the most time- 
consuming aspects of wetland operation and maintenance. 
Crites and Ogden (1998) report the operating costs for FWS 
constructed wetlands and VSBs range from $0.10 to $0.30 
and $0.04 to $0.08, respectively, per 3,785 L (1,000 gal) of 
treated water. 


At the FWS wetland system at Ouray, CO, the O&M re¬ 
quirements for the wetland are as follows: 


• Check berms for animal 
damage and erosion 

Once per week 

• Check and clean effluent 
debris screens 

Once per week 

• Observe and adjust water 
levels and flow rates 

Once per month 

• Remove sludge from inlet 
zone 

As required 

• Flush manifold pipes 

As required 

• Mosquito control 

As required by local 
health authorities 

The 1997 monthly operating costs for the complete treat 
ment system (aerated lagoon, FWS, chlorination/dechlori 
nation) at Ouray, CO, were 

• Power for lagoon aerators 

$1,400 

• Lagoon sludge removal and disposal 800 

• Miscellaneous supplies 

125 

• NPDES laboratory tests 

300 

• Wages 

1,083 

• Total 

$3,708 


125 







Table 7-10. Range of Capital Costs for a 0.4 Hectare (Membrane Lined VSB and FWS Wetland) 

Item Vegetated Submerged Bed (VSB) Free Water Surface (FWS) 



Low Cost Range 

High Cost Range 

Low Cost Range 

High Cost Range 


Cost' 

Percent 

Cost' 

Percent 

Cost' 

Percent 

Cost' 

Percent 


($/hectare) 

of Total 

($/hectare) 

of Total 

($/hectare) 

of Total 

($/hectare) 

of Total 

Survey/Geotechnic 

2,718 

1.2 

5,436 

1.3 

2,718 

1.9 

5,436 

2.1 

Clear & Grub 2 

4,942 

2.2 

12,355 

3.0 

4,942 

3.4 

12,355 

4.8 

Earthwork 3 

18,039 

8.1 

29,900 

7.3 

18,039 

12.4 

29,900 

11.7 

Membrane Liner 4 









30 mil PVC 

37,807 

16.9 

43,243 

— 

37,807 

26.1 

43,243 

— 

40 mil PE 

43,243 

— 

48,432 

— 

43,243 

— 

48,432 

— 

40 mil PPE 

53,869 

— 

59,305 

— 

53,869 

— 

59,305 

— 

45 mil Reinf. PPE 

64,494 

— 

69,931 

— 

64,494 

— 

69,931 

— 

60 mil Hypalon 

69,931 

— 

80,803 

— 

69,931 

— 

80,803 

— 

XR-5 

102,301 

— 

113,174 

27.5 

102,301 

— 

113,174 

44.4 

Media 

129,483 s 

57.8 

199,413 s 

48.5 

16,062® 

11.1 

20,015® 

7.8 

Plants & Planting 7 

8,649 

3.9 

17,297 

4.2 

8,649 

6.0 

17,297 

6.8 

Control Structures 

4,942® 

2.2 

16,062 

3.9 

39,537 9 

27.3 

39,600 

15.5 

Plumbing & Fencing 

17,297 

7.7 

17,297 

4.2 

17,297 

11.9 

17,297 

6.8 

Totals: 

233,877'° 

100 

410,935" 

100 

145,050'° 

100 

255,012" 

100 


'Adjusted to 1997 dollars (ENR CCI = 5854) 

^Clearing and grubbing costs are higher for sites with large trees 

3 Earthwork (excavation and compaction) costs are typically 2.00 to 3.25 per m 3 . A 0.9 m deep excavation was assumed. 

4 For rocky soils, add 5,435 to 8,645 per hectare to the costs presented. For a site employing a minimum of 9,300 m 2 of liner, deduct 5,435. Delete 
the costs for the liner if native clay liner is used. 

5 Reported cost range is for 60 cm of gravel media with 15 cm of pea gravel. Gravel costs typically range from 17 to 26 per m 3 within 20 miles of the 
project. Longer delivery distances will increase the cost. 

6 Assumes 15 cm of topsoil over the wetland bottom. 

Planting costs are typically 0.50 to 1.00 per plant. Values shown are for planting at 2.5 sq. ft centers. Adjust cost if different spacing is used. 

8 122 m of 100 mm diameter PVC manifold plus one water level control box 

9 Eight concrete weir box structures for inlets/outlets 

,0 Uses 30 mil PVC liner 

"Uses XR-5 liner 

0.405 ha = 1 acre 


The tasks specifically related to the wetland components 
are estimated to require about 16 hours per month or about 
$3,000 per year. On an areal basis, this equates to $3,370 
per hectare per year ($1,364/acre per year) for this 0.89- 
hectare (2.2-acre) wetland system. These wetland costs rep¬ 
resent about 7% of the total O&M costs for the entire treat¬ 
ment process. If more rigorous testing than the minimal 
NPDES testing were required, then monitoring could become 
the most expensive cost of all O&M categories shown. 

The annual O&M expenses at the VSB system in Carville, 
LA (0.15 mgd), are shown in Table 7-11. At Carville, the an¬ 
nual costs directly associated with the wetland components 
are estimated to be $650 per year or $2,500 per hectare per 
year ($1,015/acre per year). These costs are about 6% of 
the total O&M costs for the entire treatment system. 

The 1996 O&M costs for the Gustine, CA, sewer depart¬ 
ment were about $433,275, which included bond repayment, 
engineering fees, and other contractual services. A single 
major expense was $152,402 for electrical power for pumps 
and the lagoon aerators. The direct O&M costs for the sewer 
system, the lagoons, and the wetland were $280,873. Since 
the wetland O&M tasks at Gustine are similar to those previ¬ 
ously described for Ouray and Carville, it can be assumed 
that the cost percentage determined at those systems is also 
applicable at Gustine. Using a 7% factor, the annual wetland 
O&M costs at Gustine would be $19,661 or approximately 
$2,025 per hectare per year ($819/acre per year). 


At Areata, CA, it is estimated that the O&M tasks directly 
related to the wetlands require 20 minutes of operator time 
per day or 122 hours per year. The major tasks are weir 
adjustments and berm inspections. At an assumed cost of 
$30 per hour (including benefits, incidentals, and support 
costs), the annual O&M costs for the Areata wetlands would 
be $1,205 per hectare per year ($488/acre per year). 

For the small system at Ten Stones, VT (6,700 gpd), one- 
half hour per month is estimated to inspect the system and 
pumps and to adjust water levels if necessary. These efforts 
will be voluntary on the part of the corporation members, so 
there will be no actual cost for the service. However, if a 
contractual service were retained at the previously assumed 
rate of $30 per hour, the annual maintenance costs would 
be $4,043 per hectare per year ($1,636/acre per year). 

The remainder of the systems included in the 1997 EPA 
Survey (West Jackson County, MS; Mandeville, LA; Mes¬ 
quite, NV; and Sorrento, LA) did not have recorded data for 
separate wetland O&M costs, and estimates were not pro¬ 
vided. Table 7-12 summarizes the O&M cost data that was 
available. 

It is not possible to divide the total annual O&M costs shown 
in Table 7-12 into ranked categories for the major O&M tasks. 
The major O&M tasks are visual inspections of the berms 
and of plant health, and adjustments in water levels and other 
flow-control structures as required. Both nutria and musk- 


126 












Table 7-11. Annual O&M Costs at Carville, LA (570 m 3 /d) Veg¬ 
etated Submerged Bed 

Annual Cost 

Item (1997$) 


Electricity 


Lagoon aerators 

4,080 

UV disinfection 

316 

Miscellaneous 

350 

Maintenance 

Repair berms (30 man-hours) 

478 

Maintain UV system (60 man-hours) 

956 

Berm grass cutting (50 man-hours) 

797 

Parts & Supplies 

Aerators 

230 

UV System 

800 

Flow meter 

25 

NPDES Monitoring 

Labor (192 man-hours) 

3,209 

Materials 

350 

Total 

11,591 


Table 7-12. Annual O&M Costs for Constructed Wetlands 


Location 

Design Flow 
(m 3 /d) 

Treatment Area 
(hectares) 

Cost 

($/ha-yr) 

Ouray, CO (FWS) 

1375 

0.89 

3370 

Gustine, CA (FWS) 

3785 

9.71 

2025 

Ten Stones, VT (VSB) 

25 

0.05 

4045 

Carville, LA (VSB) 

565 

0.26 

2510 


’1997$ 


rats can cause physical damage and leakage in the berms 
and can destroy, as well as some insects, the plant cover in 
the wetland cells. If routine visual observations indicate 
damage, a more intense O&M effort will be necessary for 
repair and animal or insect control. 

Active mosquito control may be an issue in California 
and in the southwestern and southeastern states, and the 
FWS system O&M costs will increase accordingly. None 
of the systems listed in Table 7-12 were making special 
efforts for either animal or mosquito control. On a long¬ 
term basis, it will be necessary to remove accumulated 
sediment from the wetland cells when it begins to interfere 
with the hydraulic performance of the system. A ramp for 
this purpose should be included in the design and con¬ 
struction of each wetland cell. 

7.4 References 

Campbell, C.S. and M.H. Ogden. 1999. Constructed wet¬ 
lands in the sustainable landscape. New York, NY: John 
Wiley and Sons. 

Crites, R.W. and M.H. Ogden. 1998. Costs of constructed 
wetlands systems. Presented to WEFTEC, WEF 71st 
Annual Conference, Orlando, FL. 

Gearheart., R.A., B.A. Finney, M. Lang, and J. Anderson. 
1998. A comparison of system planning, design, and 
sizing methodologies for free water surface constructed 
wetlands. In: 6th International Conference on Wetland 
Systems for Water Pollution Control. IAWQ. 

Middlebrooks, E.J., C.H. Middlebrooks, J.H. Reynolds, G.Z. 
Watters, S.C. Reed, and D.B. George. 1982. Waste- 
water stabilization lagoon design, performance, and 
upgrading. New York, NY: Macmillan Publishing Co. 


127 












Chapter 8 
Case Studies 


In 1997, a series of site visits to constructed treatment 
wetlands was performed for the U.S. EPA to compile back¬ 
ground information and assess performance of free water 
surface (FWS) and vegetated submerged bed (VSB) wet¬ 
lands. Edited versions of these site visit reports are pre¬ 
sented in this chapter to enrich the reader’s understand¬ 
ing with insight gained from actual construction and op¬ 
eration of treatment wetland systems. Most quantifying 
terms are expressed in English units. Conversion factors 
are as follows: 

• 1 acre = 0.405 ha 

• 1 mgd = 3,780 m 3 /d 

• 1 ft = 0.3 m 

• 1 gal/day-ft 2 = 4 cm/d = 0.004 m 3 /m 2 -d 

• 1 in = 2.54 cm 

• 1 lb = 0.454 kg 

8.1 Free Water Surface (FWS) Constructed 
Wetlands 

8.1.1 Areata, California 

8.1.1.1 Background 

Areata is located on the northern coast of California about 
240 miles north of San Francisco. The population of Areata 
is about 15,000. The major local industries are logging, 
wood products, fishing, and Humbolt State University. The 
FWS constructed wetland located in Areata is one of the 
most famous in the United States. 

The community was originally served, starting in 1949, 
with a primary treatment plant that discharged undisinfected 
effluent to Areata Bay. In 1957, oxidation ponds were con¬ 
structed, and chlorine disinfection was added in 1966. In 
1974, the State of California prohibited discharge to bays 
and estuaries unless “enhancement” could be proven, and 
the construction of a regional treatment plant was recom¬ 
mended. In response, the City of Areata formed a Task 
Force of interested participants, and this group began re¬ 
search on lower-cost alternative treatment processes us¬ 
ing natural systems. From 1979 to 1982, research con¬ 
ducted at pilot-scale wetland units confirmed their capabil¬ 


ity to meet the proposed discharge limits. In 1983, the city 
was authorized by the state to proceed with development, 
design, and construction of a full-scale wetland system. 
Construction was completed in 1986, and the system has 
been in continuous service since that time. 

The wetland system proposed by the city was unique in 
that it included densely vegetated cells dedicated for treat¬ 
ment followed by “enhancement” marsh cells with a large 
percentage of open water for final polishing and habitat 
and recreational benefits. This combined system has been 
successful since start-up and has become the model for 
many wetland systems elsewhere. 

Two NPDES permits are required for system operation: 
one for discharge to the enhancement wetlands for pro¬ 
tection of public access and one for discharge to the bay. 
The NPDES limits for both discharges are BOD 30 mg/L 
and TSS 30 mg/L, pH 6.5 to 9.5, and fecal coliforms of 200 
CFU/100 mL. Since public access is allowed to the en¬ 
hancement marshes, the state required disinfection prior 
to transfer of the pond/treatment marsh effluent. The state 
then required final disinfection/dechlorination prior to final 
discharge to Areata Bay. The effluent from the final en¬ 
hancement marsh is pumped back to the treatment plant 
for this final disinfection step. 

The basic system design for the treatment and enhance¬ 
ment marshes was prepared by Dr. Robert Gearheart and 
his colleagues at Humbolt State University. The design was 
based on experience with the pilot wetland system that 
was studied from 1979 through 1982. 

The pilot wetland system included 12 parallel wetland 
cells, each 20 ft wide and 200 ft long (L:W 10:1), with a 
maximum possible depth of 4 ft. These were operated at 
variable hydraulic loadings, variable water depths, and 
variable initial plant types during the initial phase of the 
study. Hardstem bulrush (Scirpus validus) was used as 
the sole type of vegetation on all cells. The inlet structure 
for each cell was a 60° V-notch weir, and the outlet used 
an adjustable 90° V-notch weir, permitting control of the 
water depth. Heavy clay soils were used for construction 
of these cells, so a liner was not necessary and seepage 
was minimal. The second phase of the pilot study focused 
on the influence of open water zones, plant harvesting, 
and kinetics optimization for BOD, TSS, and nutrient re- 


128 



moval. Some of the cells, for example, were subdivided 
into smaller compartments with baffles and weirs along 
the flow path. The results from these pilot studies not only 
provided the basis for full-scale system design but have 
contributed significantly to the state-of-the-art for design 
of all wetland systems. 

The full-scale treatment wetlands, with a design flow of 
2.9 mgd, utilize three cells operated in parallel. Cells 1 
and 2 have surface areas of about 2.75 acres each (L * 
600 ft, W « 200 ft), and cell 3 is about 2.0 acres (L * 510 ft, 
W * 170 ft). The original design water depth was 2 ft, but 
at the time of the 1997 site visit for this report they were 
being operated with a 4-ft depth. Hardstem bulrush was 
again used as the only plant species on these treatment 
marshes. Clumps of plant shoots and rhizomes were hand 
planted on about 1-m centers. Since nutrient removal is 
not a requirement for the full-scale system, the treatment 
marshes could be designed for a relatively short detention 
time primarily for removal of BOD and TSS. The HDT in 
these three cells is 1.9 d at design flow and a 2-ft water 
depth. These treatment marshes were designed to pro¬ 
duce an effluent meeting the NPDES limits for BOD and 
TSS (30/30 mg/L) on an average basis. These wetland 
cells utilized the bottom area of former lagoon cells. A sche¬ 
matic diagram of the operating system is shown in Figure 
8 - 1 . 

The final “enhancement” marshes were intended to pro¬ 
vide for further effluent polishing and to provide significant 
habitat and recreational benefits for the community. These 


three cells are operated in series at an average depth of 
2.0 ft and have a total area of about 31 acres. Retention 
time is about 9 d at average flow rates. The first cell (Allen 
Marsh), completed in 1981, was constructed on former log 
storage area and contains about 50% open water. The 
second cell (Gearheart Marsh), completed in 1981, was 
constructed on former pasture land and contains about 
80% open water. The third cell (Hauser Marsh) was con¬ 
structed in a former borrow pit and contains about 60% 
open water. These 31 acres of constructed freshwater (ef¬ 
fluent) marshes have been supplemented with an addi¬ 
tional 70 acres of salt water marshes, freshwater wetlands, 
brackish ponds, and estuaries to form the Areata Marsh 
and Wildlife Sanctuary, all of which has been developed 
with trails, an interpretive center, and other recreational 
features. The shallow water zones in these marshes con¬ 
tain a variety of emergent vegetation. The deeper zones 
contain submerged plants (Sago pondweed) that provide 
food sources for ducks and other birds and release oxy¬ 
gen to the water to further enhance treatment. 

The construction costs for the entire system, including 
modifications to the primary treatment plant, disinfection/ 
dechlorination, pumping stations, and so forth were 
$5,300,000 (1985$). Construction costs for the treatment 
wetlands are only estimated to be about $225,000, or 
$30,000 per acre, or $78 per 1000 gpd of design capacity 
(including removal of sludge from this site, which was pre¬ 
viously a sedimentation pond for an aerated lagoon). This 
does not include pumping costs to transfer final effluent 
back to the chlorination contact basin, disinfection facili- 




Figure 8-1. Schematic diagram of wetland system at Areata, CA 


129 














ties, or the pumping and piping costs to reach the enhance¬ 
ment marshes. Land costs also are not included since the 
treatment wetlands were located on city-owned property. 

8.1.1.2 Financial Arrangements 

Construction costs for the Areata system were funded 
by a state/federal construction grant program with a grant 
for 85% of the project costs. O&M costs for the system are 
paid with a surcharge on the consumer’s water bills. 

8.1.1.3 Construction and Start-up Procedures 

Three of the final cells in the existing treatment pond 
were selected for the treatment wetlands. This allowed the 
use of city-owned land at no cost, a gravity flow connec¬ 
tion from the ponds, and clay soils that eliminated the need 
for liners and minimized the earthwork requirements. The 
lagoon cells were drained and dried, local fill was placed 
to the desired grade, and the wetland bottoms were graded 
level. Shallow drainage channels were excavated to per¬ 
mit draining of the cell if desired. Construction of inlet and 
outlet structures completed the construction activities. Each 
cell has only one inlet and outlet structure. The wetland 
effluent was collected from the bottom of the wetland in 
each of these structures. The inlet structure has an adjust¬ 
able weir so the flow to the three cells can be balanced. 
The outlet weir is not adjustable and was originally de¬ 
signed to maintain a 2-ft water depth in the cell. Prior to 
the 1997 site visit, timber sections had been bolted to the 
top perimeter of the outlet box. This raised the water level 
in the bed and converted this box to a four-sided overflow 
weir. This allowed wetland effluent to be decanted from 
the top of the wetland rather than off the bottom. The new 
4-ft water depth was intended to suppress undesirable plant 
species that had started to spread in the wetland. 

The treatment wetland cells were hand planted with 
hardstem bulrush with clumps of shoots and rhizome ma¬ 
terial planted on about 1-m centers. These plants were 
obtained from the pilot wetland channels. The wetland cells 
were flooded to a very shallow depth with tap water for 
about three months to encourage new plant development 
and growth. Wastewater was not applied at full depth until 
the new plants were 3- to 4-ft tall and construction of the 
rest of the system was complete (that is, effluent pump 
station and other features). 

The enhancement marshes were constructed on avail¬ 
able waterfront land but at some distance from the basic 
treatment system, so a pumping station and transmission 
piping were required. These enhancement wetlands were 
also sited on clay soils, so extensive soils and geotechnical 
investigations were not necessary. The grading for these 
wetlands was more complex than the treatment wetlands 
because berms did not previously exist, and it was de¬ 
sired to produce a wetland with different water depths and 
with several nesting islands in each cell. Each of these 
cells also contains a single inlet and a single outlet struc¬ 
ture. The final effluent comes through a highly vegetated 
zone of emergent macrophytes with no open water. The 
final cell is followed by a pumping station to return effluent 
to the treatment plant for final disinfection and discharge. 


The entire treatment system is operated and maintained 
by three operators who work five days per week and are 
on call on weekends. The only wetland-related O&M task 
is adjustment of the inlet weirs to ensure that flow is prop¬ 
erly balanced and to visually observe the status of the treat¬ 
ment wetlands; this might require 20 minutes per day. With 
a total O&M effort of 87 hours per year at an assumed rate 
of $30/hr for operator costs, the O&M costs for the treat¬ 
ment wetlands alone would be $2,600 per year. The O&M 
costs for the pumping and the double chlorination would 
add significantly to that, but these are unique to the Areata 
system and not generic to all wetland systems. Harvesting 
or other special vegetation-management activities are not 
practiced at this site. 

8.1.1.4 Performance History 

Performance data were collected for a two-year period 
during the Phase 1 pilot testing program. This program 
varied the flow rate and water depth in each of the two 
cells to compare BOD removal performance at different 
detention times and loading rates that would represent the 
potential range for full-scale application at Areata. These 
data are summarized in Table 8-1. The BOD and TSS in 
the pond effluent varied considerably during this period, 
and not all of the cells were uniformly vegetated. Seasonal 
variations in performance were observed, but Table 8-1 
presents only the average effluent characteristics for each 
of the cells over the entire study period. It is apparent from 
the data that the wetlands were able to produce excellent 
effluent quality over the full range of loadings and deten¬ 
tion times used. 

The long-term average performance of the Areata sys¬ 
tem is summarized in Table 8-2. It is clear that both the 
treatment and enhancement marshes provide significant 
treatment for BOD and TSS. The long- term removals fol¬ 
low the pilot project results. Most of the nitrogen is removed 
during the final stage in the enhancement marshes. This 
is because of the long hydraulic detention time (HRT = 9 
d), the availability of oxygen and nitrifying organisms in 


Table 8-1. Summary of Results, Phase 1 Pilot Testing, Areata, CA 
(cells 1-4 had received two different hydraulic loading rates for one 
year each-the higher loading occurred the first year) 


Item 

HRT 

HLR 

B0D 5 

TSS 

FECAL COLI 


(Actual) 

gal/ft 2 -d 

mg/L 

mg/L 

CFU/100 ml 

Influent 


26 

37 

3183 


Effluent: 






Cell 1 

2.1/10.7 

5.89/1.22 

11 

6.8 

317 

Cell 2 

1.5/17 

5.89/0.5 

14.1 

4.3 

272 

Cell 3 

2.7/29 

4.66/0.5 

13.3 

4.7 

419 

Cell 4 

1.5/15 

5.39/0.5 

12.7 

5.6 

549 

Cell 5 

3.7 

2.94 

14.0 

4.3 

493 

Cell 6 

5.2 

2.4 

10.7 

4.0 

345 

Cell 7 

5.2 

4.4 

13.3 

7.3 

785 

Cell 8 

5.2 

2.4 

15.3 

7.2 

713 

Cell 9 

6.6 

1.71 

11.9 

9.4 

318 

Cell 10 

3.8 

1.71 

12.6 

4.9 

367 

Cell 11 

7.6 

1.47 

9.4 

5.7 

288 

Cell 12 

5.5 

1.47 

9.0 

4.3 

421 


130 






Table 8-2. Long-Term Average Performance, Areata WWTP 


Location 

BOD 

TSS 

TN 


mg/L 

mg/L 

mg/L 

Raw Influent 

174 

214 

40 

Primary Effluent 

102 

70 

40 

Pond Effluent 

53 

58 

40 

Treatment Wetlands 

28 

21 

30 

Enhancement Marshes 

3.3 

3 

3 


the open water zones, and anoxic conditions for denitrifi¬ 
cation in the areas with emergent vegetation. 

8.1.1.5 Lessons Learned 

The treatment wetlands (7.5 acres), with nominal HRTs 
of three days, met weekly limits of 30 mg/L BOD and TSS 
90% of the time. The enhancement wetlands (28 acres), 
with a nominal HRT of 11 days, met weekly limits of less 
than 5 mg/L BOD/TSS 90% of the time. Performance of 
both wetlands results primarily from proper operation and 
appropriate design that involves a combination of emer¬ 
gent vegetation and open water zones. TSS levels are 
higher in cell effluents where outlets are located in open 
water zones. 

Wetland habitat values and opportunities for research 
and environmental education provided by the enhance¬ 
ment marshes were optimized to gain state approval for a 
near-shore discharge to Areata Bay. Optimizing ancillary 
benefits appears also to have complemented treatment 
capabilities. 

Nitrogen leaving the treatment wetlands in the ammonia 
form is nitrified in the open water zones in the enhance¬ 
ment marshes, where deeper open water zones with sub¬ 
merged stands of Sago pond weed (Potemogeton 
pectinatus) produce oxygen, and plant surfaces become 
the substrate for attached- growth nitrifying organisms. This 
plant also offers important habitat values since it is a ma¬ 
jor food source for many duck species. Long total deten¬ 
tion times (~9 d) and alternating open and vegetated zones 
resulted in excellent nitrogen-removal performance. 

Duckweed that grows on the open water surfaces is pre¬ 
vented from becoming a permanent duckweed mat by suf¬ 
ficient wind action. In vegetated areas, duckweed does 
mat, and it impedes reaeration. 

Denitrification takes place in the fully vegetated anoxic 
zones in the enhancement marshes. 

Most of the fecal coliforms in the effluent are from birds 
and other wildlife in the marshes and not from wastewater 
sources. 

Pilot-scale marshes produced better treatment than the 
full-scale treatment wetlands, possibly due in part to the 
different configurations of the two systems and the possi¬ 
bility for short-circuiting in the larger full-scale cells. A 3:1 


aspect ratio for the full-scale cells is acceptable as long as 
the influent is uniformly distributed over the full width of 
the cell and the effluent collected in a comparable man¬ 
ner. 

Short-circuiting probably occurs in the treatment cells, 
but this could be corrected by replacing the single-point 
inlet and outlet structures with perforated pipe manifolds 
extending the full width of the cell. 

8.1.2 West Jackson County, Mississippi 
8.1.2.1 Background 

The West Jackson County (WJC) wastewater treatment 
system is owned and operated by the Mississippi Gulf 
Coast Regional Wastewater Authority. It is one of several 
treatment systems serving communities within the 
Authority’s boundaries. The system is located near Ocean 
Springs, MS, on the north side of 1-10, about 20 miles east 
of Biloxi, Mississippi. 

The original WJC system included a 75-acre, multiple¬ 
cell facultative lagoon for preliminary treatment followed 
by 415 acres of slow-rate (SR) land treatment fields (grow¬ 
ing hay). The land treatment site was underdrained, and a 
portion of that recovered water was to be used to supple¬ 
ment dry-weather flow into marshes in the Mississippi San¬ 
dhill Crane National Wildlife Refuge. This system com¬ 
menced operation in October 1987 with a design flow rate 
of 2.6 mgd and a 56 d HRT. Problems developed soon 
after start-up since the clay soils at the land treatment site 
proved not to be as permeable as originally expected. Af¬ 
ter extensive investigations and discussions, the design 
consultant agreed to design and construct a supplemental 
free water surface wetland to treat the excess flow. 

Three parallel wetland units were constructed with a to¬ 
tal area of 56 acres to treat a design flow of 1.6 mgd. The 
discharge from this new wetland was to Castapia Bayou, 
with NPDES permit limits for BOD at 10 mg/L, TSS at 30 
mg/L, NH 4 -N at 2 mg/L, DO at 6 mg/L, pH at 6.0 to 8.5, and 
fecal coliforms at 2200/100 mL. Phase 1 of this new wet¬ 
land was placed in operation in 1990 and Phase 2 in 1991. 
Figure 8-2 is a schematic diagram of the 56-acre wetland. 
Detailed costs are not available for the Phase 1 and 2 
wetlands since they were funded privately by the design 
consultant as part of the agreement with the Wastewater 
Authority. Costs for Phase 3 (2.4 mgd) are available. 

The Phase 1 wetland had two cells in series totaling 22 
acres. The Phase 2 wetland (set 2) had three cells totaling 
21.5 acres and another (set 3) which had two cells with a 
total area of 12.5 acres. Local soils were all clays, so a 
liner for these wetlands was not required. Bottoms of all 
wetland cells were constructed with an average slope of 
0.19% in the flow direction. At the end of each cell, mul¬ 
tiple weir boxes were used as outlet structures with ad¬ 
justable weir plates, allowing a maximum water depth in 
the outlet zone of up to 2 ft. At the mean depth of 0.75 ft, 
the design hydraulic residence times in the three wetlands 
were 12.5 d in Phase 1,10.1 d in Phase 2-2, and 10.7 d in 


131 










Post Aeration 


Phase 2 


Cell 

Area-Acres 

(hectares) 

1A 

12 

(4.8) 

IB 

10 

(4.0) 

2A 

10 

(4.0) 

2B 

8 

(3.2) 

2C 

4 

(1-6) 

3A 

9 

(3.6) 

3B 

3 

(1.2) 


<4 



Figure 8-2. Schematic diagram of Phase 1 and 2 wetland systems at West Jackson County, MS 


Phase 2-3. The inlets to all three sets of cells used a 12- 
inch perforated PVC pipe manifold. The elevation of the 
Phase 1 wetland was slightly higher than the lagoon, so it 
was necessary to pump lagoon effluent to this wetland. 
The Phase 2 wetlands were at a lower elevation, and gravity 
was used as the motive force. 

A unique feature of this wetland system was the incor¬ 
poration of “deep zones” in each wetland cell. These con¬ 
sisted of trenches excavated perpendicular to the flow di¬ 
rection, with the bottom of the trench excavated about 5 ft 
below the general wetland bottom surface. This provided 
a water depth of about 6 ft in these “deep zones,” which 
was sufficient to prevent colonization by the emergent 
wetland plants. These trenches are about 20 ft wide at the 
bottom and about 40 ft wide at the top. The purpose of 
these “deep zones” was to redistribute the flow across the 
width of the cell to minimize short-circuiting and to provide 
an open water surface for atmospheric reaeration to sup¬ 
ply the oxygen necessary for ammonia removal. The po¬ 
tential open water provided by these zones was only about 
10% of the surface area in each wetland cell. The water 
surface in these zones also quickly became colonized by 
duckweed (Lemna spp.). 

In larger open water bodies, duckweed is very suscep¬ 
tible to wind action; as a result, the water surface can re¬ 


main available for atmospheric reaeration. At this location, 
with the relatively narrow “deep zones” and the protection 
provided by the adjacent emergent vegetation in the shal¬ 
low portions of the marsh, wind was not sufficient to move 
the duckweed mat, and oxygen transfer from the atmo¬ 
sphere did not develop. 

Because there was insufficient oxygen in the system to 
continuously support nitrification reactions, there were sea¬ 
sonal violations (particularly in late summer and early fall) 
of the ammonia limits commencing in 1992. Corrective 
action for this problem considered an external vertical-flow 
filter bed for nitrification and submerged tubing aeration in 
the “deep zones” to provide the necessary oxygen. The 
latter was selected as the lower-cost alternative and was 
installed in 1993. Problems again developed because nu¬ 
tria (an animal similar to a muskrat), which occupy the 
wetland in large numbers, were attracted to the air bubbles 
and destroyed the aeration tubing by gnawing on it. In sub¬ 
sequent discussions with the State of Mississippi, it was 
decided that the ammonia limit would not be enforced until 
the Castapia Bayou began to exhibit oxygen stress, so the 
aeration tubing was not replaced. 

The population is increasing rapidly in the communities 
served by this system, and by 1996 the average flow into 
these wetland units had reached 2.2 mgd. The Wastewa- 

































ter Authority then authorized an upgrade and expansion of 
the facility for a design flow of 5 mgd (1 mgd to land treat¬ 
ment, 4 mgd to wetlands). The design was completed in 
1996 and construction began in August 1997. The expan¬ 
sion included modifications to the lagoon (providing aera¬ 
tion and baffles), 50 acres of additional wetland area, a 
plastic-media trickling filter bed for nitrification, UV disin¬ 
fection, and additional post-treatment aeration to ensure 
adequate DO in the final effluent. The expanded wetland 
system is shown in Figure 8-3. The trickling filter compo¬ 
nent has been designed but will not be constructed until 
the State of Mississippi decides it will be necessary. Re¬ 
cent data, shown in Figure 8-3, indicate that the open wa¬ 
ter zones appear to be functioning well. A UV disinfection 
system was added to the system because the NPDES 
permit limits for fecal conforms have been modified to 200/ 
100 mL in the summer and 2000/100 mL in the winter. 

Additional features of the existing Phase 1 and Phase 2 
wetlands include post-treatment aeration to satisfy the 
NPDES discharge limit for dissolved oxygen (6 mg/L). 
Multiple outlet structures with adjustable weirs are used 
for cell-to-cell transfer and for final discharges from the 
wetlands. A miniature “deep zone” was excavated around 
each of these structures to prevent the growth of emer¬ 
gent vegetation in the immediate vicinity, as done for a 
FWS system at Fort Deposit, AL. Published design mod¬ 


els were not used in this case, and effluent quality (Figure 
8-3) is excellent. 

8.1.2.2 Financial Arrangements 

The initial lagoon/land treatment system was funded 
under a federal/state construction grant program in exist¬ 
ence at the time. The Phase 1 and 2 wetlands were funded 
directly by the design consultant. The Phase 3 expansion 
was funded through a revolving loan fund as administered 
by the State of Mississippi. Total costs for the entire Phase 
3 project are estimated to be $2,758,000 (1997$). This 
includes lagoon modifications, UV disinfection, the nitrifi¬ 
cation trickling filter, and post-aeration. The costs for just 
the 50 acres of wetland expansion are estimated to be 
about $700,000, or $14,000 per acre, or $250 per 1000 
gallons of design flow capacity. Land costs for this project 
are not included in this estimate since the land already 
belonged to the Wastewater Authority. The O&M costs are 
funded by a surcharge on each consumer’s water bill within 
the Authority’s service area. 

8.1.2.3 Construction and Start-up Procedures 

The site for the original lagoon and land treatment site 
was selected for its proximity to the Mississippi Sandhill 
Crane National Wildlife Refuge, where the treated effluent 
could be utilized in refuge marshes. The sites for the Phase 



Figure 8-3. Schematic diagram of Phase 3 wetland expansion at West Jackson County, MS 


133 




















































1 and 2 wetlands were selected to take advantage of avail¬ 
able land already owned by the Wastewater Authority and 
for proximity to Castapia Bayou for the system discharge 
point. As shown in Figure 8-3, the Phase 3 wetlands were 
then located to utilize the remaining land available on this 
site. 

The site investigation for the wetlands included a 
geotechnical investigation which indicated that ground 
water impacts or intrusion would be minimized by underly¬ 
ing clay subsoil. Borings and test pits verified the pres¬ 
ence of these clay soils throughout the proposed wetland 
area. Wetland sites originally were covered with scrub pine 
and related ground cover, so clearing and grubbing of the 
site was the first construction task. This was followed by 
excavation to grade with typical highway construction 
equipment and construction of both the external and inter¬ 
nal berms with spoil material from the excavations. A liner 
was not necessary due to the low permeability of clay soils, 
but geotextiles were used adjacent to the inlet and outlet 
structures to prevent erosion. The multiple concrete outlet 
and transfer structures were cast in place. 

The bulrush {Scirpus spp.) and cattails (Typha spp.) se¬ 
lected as the vegetation for this wetland system were ob¬ 
tained locally by cleaning drainage ditches within the 
Authority’s jurisdictional area. The plants were brought to 
the site and separated, and shoots were cut to about 1 ft in 
length and planted by hand in individually augured holes. 
The plants were placed on about 1 -m centers. At that den¬ 
sity, it would require about 227,000 plants to cover the origi¬ 
nal 56-acre wetland site. Observations at the time showed 
that two laborers could prepare and plant about 1,200 
plants per day. At that rate it would require about 54 man¬ 
hours per acre to vegetate a wetland bed using this tech¬ 
nique. 

As soon as a zone was planted, it was flooded with a 
very shallow depth of stream water to encourage plant 
growth. Lagoon effluent at the design depth was not ap¬ 
plied for at least 30 days after planting was completed. A 
pattern was adopted for Phase 1, on which alternating 
bands of bulrush and cattails were planted. This was aban¬ 
doned for Phase 2, so whichever species was available 
on a given day was planted; as a result, cattails were the 
dominant species in Phase 2. 

8.1.2.4 Performance History 

At the original 1.6-mgd wetland design flow rate, the 
flow was split between the three wetland units: 0.6 mgd to 
Phase 1,0.65 mgd to Phase 2-2, and 0.35 mgd to Phase 
2-3. During the period 1992 to 1995, the average effluent 
characteristics from the facultative lagoon were BOD 31 
mg/L, TSS 33 mg/L, TKN 12.9 mg/L, and NH 4 -N 4.4 mg/L. Dur¬ 
ing this same period, the combined final effluent from the wet¬ 
land units met all NPDES limits on an annual average basis: 
BOD 7.5 mg/L, TSS 4.6 mg/L, and NH 4 -N 1.85 mg/L. On a 
monthly basis, there were excursions; the BOD exceeded 
permit limits eight times (18%) and ammonia exceeded lim¬ 
its 11 times (25%). The BOD violations were randomly dis¬ 


tributed throughout the period and generally reflected 
higher-than-normal loading. A more specific pattern was 
shown by the ammonia, with the violations occurring in 
late summer and early fall. By 1996 the flow rate to the 
wetlands had increased to 2.35 mgd (47% higher than the 
original 1.6 mgd design), and the excursions for both BOD 
and NH 4 -N were more frequent. Table 8-3 summarizes per¬ 
formance data for the period January 1996 through July 
1997. It would appear from the data in this table that BOD 
removal is slightly better in the warmer months, indicating 
some dependence. 

8.1.2.5 Lessons Learned 

The inlet zone was submerged with a significant depth 
of water, so the overland flow mode with very shallow sheet 
flow did not develop. Ammonia removal provided by the 
shallow sheet flow of water and the continuous availability 
of oxygen intended for the system did not take place, even 
with the weirs at their lowest setting. 

“Deep zones” in each cell were intended to provide ad¬ 
ditional oxygen to support nitrification reactions, but this 
benefit was not realized when duckweed mats formed over 
the surface of the deep zones. A single large “deep water” 
zone in each cell, instead of multiple narrow trenches, 
should have allowed sufficient duckweed movement to 
sustain atmospheric reaeration. 

The Phase 3 design provides both a perforated mani¬ 
fold and an open ditch “deep zone” in the inlet area of the 
cells to promote proper lateral distribution of the influent 
and increased volume to capture incoming TSS. 

Bulrush plants had been almost completely removed by 
muskrats and nutria using these plants for food and nest¬ 
ing material, but damage to cattails was minor. Damaged 


Table 8-3. Wetland Water Quality, West Jackson Co., MS 


Date 

bod 5 , 

In 

mg/L 

Out 

TSS, mg/L 

In Out 

Nl-L- 

4 

In 

-N mg/L 
Out 

1996 

Jan 

36 

10 

28 

12 

7 

4 

Feb 

32 

13 

21 

10 

12 

4 

Mar 

36 

12 

24 

14 

9 

6 

Apr 

30 

14 

18 

6 

9 

6 

May 

32 

8 

32 

5 

10 

4 

Jun 

38 

6 

38 

3 

18 

4 

Jul 

36 

3 

42 

5 

6 

3 

Aug 

37 

9 

60 

12 

12 

2 

Sep 

32 

8 

40 

13 

2 

2 

Oct 

34 

8 

90 

7 

2 

1 

Nov 

34 

8 

41 

5 

2 

1 

Dec 

20 

10 

17 

3 

1 

1 

1997 

Jan 

33 

13 

18 

5 

1 

1 

Feb 

23 

9 

17 

6 

2 

2 

Mar 

43 

10 

24 

5 

16 

8 

Apr 

41 

9 

23 

4 

9 

6 

May 

28 

4 

24 

2 

9 

7 

Jun 

47 

6 

23 

2 

1 

2 

Jul 

42 

4 

43 

3 

3 

2 


134 










sections in Phase 1 were planned to be temporarily drained 
and replanted with cattails. In the new Phase 3 addition, 
cattails were proposed as the only plant in the bottoms, 
and a variety of attractive flowering wetland species were 
planned for the inside perimeter of the new cells. 

The system supports large numbers of birds and other 
wildlife, even though special measures to enhance habitat 
values were not taken in Phases 2 and 3. 

Birds and other wildlife in the wetlands appear to have a 
significant impact on effluent fecal conforms from the sys¬ 
tem, with the final wetland effluent often higher than la¬ 
goon effluent entering the wetland. 

8.1.3 Gustine, California 

8.1.3.1 Background 

Gustine is an agricultural community located in the Cen¬ 
tral Valley of California on the east side of 1-5 and about 60 
miles south of Stockton. There are several milk-process¬ 
ing industries in the community that impose high organic 
loadings on the municipal wastewater treatment system. 
The original treatment system consisted of an oxidation 
pond with 14 cells operated in series (HRT * 56 d, aver¬ 
age pond depth ~ 4 ft), with final discharge without disin¬ 
fection to a small stream. Approximately one-third of the 1 
mgd design flow originates from domestic and commer¬ 
cial sources; the remaining two-thirds come from dairy prod¬ 
uct industries. This combination produces a high-strength 
wastewater with an average BOD of about 1200 mg/L and 
TSS of 450 mg/L, and these characteristics resulted in fre¬ 
quent violations of the 30 BOD/30 TSS NPDES discharge 
limits for the original lagoon system. 

In 1981, a Facility Plan for the city was funded under the 
Clean Water Act. This plan considered a number of alter¬ 
natives for upgrading the existing treatment system. The 
most cost-effective alternative was assumed to be a facul¬ 
tative lagoon followed by a constructed free water surface 
(FWS) wetland for final polishing to consistently meet the 
NPDES discharge limits. Since design criteria for FWS 
wetlands were not well established in 1981, a pilot test to 
develop final design criteria was recommended. 

A pilot study was approved and was conducted from 
December 1982 to October 1983. The pilot system modi¬ 
fied an existing ditch that already contained a stand of cat¬ 
tails. The pilot cell was 39 ft wide and 900 ft long. Partially 
treated water was taken from various intermediate pond 
cells. The influent to the pilot wetland averaged 180 mg/L 
BOD and 118 mg/L TSS. At the operational water depth of 
6 in, the HRT in the wetland averaged 2.5 d. The BOD and 
TSS in the wetland effluent stabilized at 30 mg/L after the 
start-up period. Just prior to the pilot testing, one of the 
dairy industries closed and was not expected to reopen. 
This resulted in lower-strength wastewaters than had been 
previously experienced, and these were expected to pre¬ 
vail in the future. The pilot results were used as the basis 
for the design and sizing of the full-scale wetland compo¬ 
nent. Construction of the system commenced in March 
1986 and was completed in October 1987. 


Three of the 14 existing lagoon cells (plus some addi¬ 
tional adjacent land) were selected as the site for the new 
wetland component. This area was converted to 24 wet¬ 
land cells operating in parallel. Each cell had a net area of 
about 1 acre and was 38 ft wide and 1107 ft long (L:W = 
29:1), similar to the size and the configuration of the pilot 
wetland unit. Internal berms constructed to separate the 
wetland cells were 10 ft wide and 2 ft deep. An exterior 
levee 6.5 ft high was constructed around the entire wet¬ 
land area to provide protection from the hundred-year flood, 
as required by the State of California. Influent flow from 
the lagoon passes through a distribution box where V-notch 
weirs divide the flow into six equal parts. Each part of the 
flow is then piped to a group of four cells. Gated aluminum 
pipe is used to distribute flow across the width of each cell. 
In order to provide flexibility for high-strength flows, a simple 
step-feed arrangement was designed. Pipe manifolds were 
located at the inlet to each cell and at the one-third point 
along the flow path. Each manifold was valved so the op¬ 
erator could vary the amount of flow applied to each and 
thereby avoid an overloaded inlet zone. An adjustable out¬ 
let weir at the end of each cell allows a water depth rang¬ 
ing from 4 to 18 in. These weirs discharge to a common 
sewer, and the effluent is then pumped to the chlorine dis¬ 
infection/dechlorination system. At the design flow of 1 mgd, 
the design projected an average HRT at higher loading of 
about seven days, which could be varied from 4 d in the 
summer to 11 d in the winter, depending on the number of 
cells in operation and on the water depth used. This op¬ 
erational flexibility allowed for each cell to be taken out of 
service each summer for vegetation management or other 
O&M, if required. The system is schematically depicted in 
Figure 8-4. 

At the time this system was designed, the capability to 
effectively remove algae in FWS wetland systems was not 
clearly established. This issue was a concern since very 
high concentrations of algae were known to develop dur¬ 
ing the summer months in some of the lagoon cells at 
Gustine. In order to provide the operator some control over 
this situation, the new design incorporated separate outlet 
structures in each of the last seven cells of the remaining 
11-cell (in series) lagoon system. In this way, the operator 
could visually observe which cell(s) had the least amount 
of algae present and select those for discharge to the wet¬ 
land. 

Soon after the 1987 system start-up, the milk-process¬ 
ing industry in Gustine that had been closed for several 
years was reopened and full-scale operations commenced. 
This imposed a higher than expected organic load on the 
treatment system; as a result, the lagoon/wetland system 
could not consistently meet the 30/30 (BOD/TSS) NPDES 
discharge limits, especially during the winter months. Fol¬ 
lowing consent decree discussions with the U.S. EPA, the 
City of Gustine evaluated the performance of the system 
and recommended action that would bring the system into 
compliance. The major system modification resulting from 
this study was the addition of floating aerators to most of 
the lagoon cells in order to reduce the organic loading. 


135 







a) Overall System - Plan 



b) Typical Wetland Cell - Plan 



Figure 8-4. Schematic diagrams of the wetland system at Gustine, CA 


The shallow 4-ft depth of the lagoon cells is not desirable 
for efficient aeration but was too expensive to modify. As¬ 
pirator aeration equipment was selected for this project 
because of the shallow water depth. This equipment was 
installed in 1992, and the lagoon has performed accept¬ 
ably since. 

8.1.3.2 Financial Arrangements 

Federal and state funding for the facility plan, the pilot 
study, and the design and construction of the wetland sys¬ 
tem was provided under the Clean Water Act Construction 
Grant Program administered in the early 1980s. The total 
construction cost for this wetland system was $882,000 
(August 1985$). This includes the cost of the multiple-pond 
outlet structures and related piping and the 6.5 ft levee 
around the perimeter of the wetland area. On an area ba¬ 
sis (24 acres of treatment area), the cost would be $36,750 
per acre. On a design flow basis (1 mgd system), the cost 
would be $882 per 1000 gpd of treatment capacity. Land 
costs are not included since the area was already owned 
by the city. The gross area utilized was about 36 acres for 
the wetlands, levees, and disinfection facilities. 


For this facility, O&M costs are funded by a surcharge 
on the consumer’s water bill. The City of Gustine budgeted 
$433,275 in 1996 for operation and maintenance of the 
wastewater treatment and sewerage systems in the com¬ 
munity. Based on a 1 -mgd design flow, the unit costs would 
be $1.19 per 1000 gallons treated. The O&M costs for just 
the wetland component are minimal and might represent 
less than 10% of the total (e.g., $0.12/1000 gpd). 

8.1.3.3 Construction and Start-up Procedures 

There were no soils or geotechnical or ground water in¬ 
vestigations at this site prior to or during construction. The 
local soils were clays and the existing lagoons are unlined, 
and it could therefore be assumed that the wetland cells 
would not require lining. The site lay within the flood plain 
of a small local stream, and the State of California did re¬ 
quire a 6.5-ft high levee to protect the new wetland system 
from the hundred-year flood. 

Construction commenced with the draining and drying 
of the three existing lagoon cells and clearing and grub¬ 
bing of the adjacent land required to complete the system. 


136 


































































































The entire wetland area was excavated to grade, with a 
flat bottom, and then the interior berms were placed as 
fill. Construction of the inlet and outlet structures and 
the related piping completed the physical aspects of the 
wetland system. 

Cattails (Typha latifolia) and tristar bulrush (Scirpus 
californicus) were selected for use on this wetland 
project. The specifications required that 18 of the cells 
would be planted with cattail rhizomes on 3-ft centers, 
and the remaining six cells would be planted with bul¬ 
rush rhizomes on 1.5-ft centers. During the first plant¬ 
ing attempt in September 1986, rhizomes of both plant 
species were obtained at local natural stands and spread 
on the wetland surfaces and disked into the soil. Water 
was not available for irrigation and very few plants 
emerged the following spring. The second planting at¬ 
tempt occurred in June 1987 and consisted of mechani¬ 
cal planting of cattail seedlings obtained at a nursery. 
The bed was then flooded with high-BOD pond effluent. 
Almost all of the plants died in a short time. It is be¬ 
lieved this was due to heat stress (the air temperatures 
were 100%F) and the high oxygen demand from the 
poorly treated water used to irrigate. The contractor also 
seeded the wetland area by broadcasting mixed bul¬ 
rush seed (hardstem and tristar). Some live cattail plants 
were also transplanted to the wetland beds from local 
drainage ditches. These more mature plants survived, 
whereas the small seedlings did not. By the fall of 1987, 
a few of the cells were almost completely covered with 
bulrush plants, but the majority contained random stands 
of bulrush and isolated patches of cattails. 

As a result of these planting problems, the system 
started up in 1987 with insufficient plant cover to pro¬ 
vide the necessary substrate for treatment and an or¬ 
ganic loading that was more than double the design load. 
In the spring of 1988, about half of the cells had moder¬ 
ately dense growth over about 75% of the cell area. The 
other half of the system contained only random patches 
of bulrush and cattails. This situation improved slowly 
during subsequent years; at the time of the 1989-1990 
wetland evaluation, the wetland cells were still not com¬ 
pletely covered with vegetation. 

The hardstem and tristar bulrush gradually spread and 
became the dominant species on the system. The wet¬ 
land could be considered to be completely vegetated 
since early 1993. However, during the 1997 site visit for 
this report, patches of sparse vegetation and some open 
water areas were still observed on some cells. As the 
density of the vegetation increased, it began to create 
hydraulic problems for operation of the system. Flow 
through these FWS wetlands is thought to be governed 
by Manning’s equation. The frictional resistance to flow 
through a wetland bed is significantly higher than in a 
normal grassed drainage channel since the vegetation 
(and litter) exists throughout the full depth of the water 
column. This resistance obviously increases and the 
length of the flow path increases. A system with a high 


aspect ratio (29:1 at Gustine) has the potential to de¬ 
velop a high enough resistance to force the water level 
at the inlet to increase very significantly in order to pro¬ 
vide the necessary hydraulic gradient. This occurred at 
Gustine, and the water level at the inlets overtopped the 
shallow berms. The operator had two options to solve this 
problem: increase the height of the berms or reduce the 
resistance to flow. He chose the latter course and got per¬ 
mission to burn the vegetation in late fall after senescence. 
This immediately solved the hydraulic problem, and burn¬ 
ing has become an annual occurrence at Gustine. 

There were no special start-up procedures used at this 
system, and pond effluents at the full rate were applied to 
all of the wetland cells regardless of vegetation coverage 
in the fall of 1987. However, until corrective action was 
taken in 1992 to reduce the organic loading in the ponds, 
the system frequently did not meet the NPDES discharge 
limits. 

8.1.3.4 Performance History 

As described in the previous section, this system did not 
consistently meet the NPDES limits at start-up and for sev¬ 
eral years thereafter. The problem was due to a higher 
than expected organic load (particularly in the winter 
months) and the lack of significant vegetation coverage 
on most of the wetland cells. The vegetation and litter in 
these FWS systems serve as the means for enhanced floc¬ 
culation and sedimentation that actually perform the treat¬ 
ment. The importance of this vegetation and litter can be 
seen by comparing the data in Table 8-4. These are per¬ 
formance results obtained during the 1989-1990 winter in 
the special evaluation study, when several cells in the 
Gustine wetlands were isolated and a careful performance 
evaluation was conducted over a one-year period. One 
cell (#6D in their set) was almost completely vegetated 
with bulrush; the other cell (#2A in their set) listed in Table 
8-4 was sparsely vegetated with some bulrush and cat¬ 
tails. The influent BOD was slightly different for the two 
cells because different source ponds were in use, but dur¬ 
ing the period of concern the influent values were in the 
same range. The HRT during this period was about 10 d 
for both cells. 

This FWS wetland system has had problems in meeting 
its NPDES discharge limits since start-up. During the pe¬ 
riod 1987 to 1992, the problem was believed to be an or¬ 
ganic overload on both the lagoon pretreatment and on 
the wetland component in this system as evidenced by 
the data in Table 8-4. The system design expected a BOD 
concentration of 150 mg/L entering the wetland, but the 
actual average wetland influent BOD during this initial pe¬ 
riod was close to 300 mg/L, with weekly excursions up to 
630 mg/L during the winter months. This problem was com¬ 
pounded by the immature vegetative growth in the wet¬ 
land cells that significantly reduced the flocculation/sedi¬ 
mentation treatment potential. The organic loading prob¬ 
lem was corrected by the addition of aeration capacity to 
the lagoon component, and the plant density has gradu¬ 
ally increased on the wetland cells. 


137 







Table 8-4. Performance Results in Mature Vegetated vs Immature Vegetated FWS Cells, Gustine, CA 


Full Vegetation Partial Vegetation 

BOD 5 , mg/L TSS, mg/L BOD 5 , mg/L TSS, mg/L 

Month Temp In Out In Out In Out In Out 

1989-90 °F* 


Nov 

50 

54 

8 

72 

13 

49 

20 

80 

24 

Dec 

43 

154 

17 

103 

6 

150 

66 

104 

42 

Jan 

44 

525 

35 

116 

24 

515 

153 

113 

71 

Feb 

45 

483 

29 

79 

19 

478 

123 

86 

49 

Mar 

56 

215 

16 

100 

17 

185 

21 

94 

20 

Avg 

- 

286 

21 

94 

16 

275 

77 

95 

41 


* Average water temperature in the marsh cells 


Table 8-5 presents current wetland effluent BOD and TSS 
values for 1996-1997. Ammonia (NH3) and Kjedahl nitro¬ 
gen (TKN) values also were measured during the first half 
of 1996 and are shown in Table 8-5. Wetland influent char¬ 
acteristics were not measured during this period, but the 
new aerators in the lagoon cells were operating continu¬ 
ously, so it can be assumed that the organic loading on 
the wetland probably does not exceed the original design 
expectations. During 1996 the average daily flow into the 
treatment system was 1.02 mgd, which is essentially equal 
to the design capacity, so the system is not overloaded 
hydraulically. It is clear from these data that this wetland 
system is still having difficulty meeting the NPDES dis¬ 
charge limits. The effluent BOD exceeded 30 mg/L twice 
during 1996 and once during the first half of 1997; the ef¬ 
fluent TSS met the 30 mg/L limit three times during 1996 
(25% of the time) and three times during the first half of 
1997 (50% of the time). 

8.1.3.5 Lessons Learned 

Poor performance evident in the 1996-1997 data may 
not have been caused by an organic overload, since the 
effluent BOD had been significantly below the discharge 
limit most of the time, and the few BOD excursions were 
relatively small and occurred mostly during the warm 
months. Birds and other wildlife may be a contributing fac¬ 
tor. Detritus and similar natural organic materials may also 
be a source for the excess TSS. 

Plant litter allowed to accumulate in the cells may im¬ 
prove water quality. With the litter burned each year, sol¬ 
ids entrapment must depend on living plants. Also, the 
tristar bulrush that dominates many of the wetland cells 
has narrow stalks and no leaves, so the plants’ surface 
area beneath the water surface is minimal, further reduc¬ 
ing entrapment of solids. However, plant litter in the chan¬ 
nels caused hydraulic failure when resistance to flow in¬ 
creased and the cells overflowed their banks at the entry 
zone, owing to the excessive L:W ratio. 

The hydraulic problem was corrected in 1995 when three 
interior berms in each set of four cells were removed and 
some of the surplus material was used to increase the 
height of the remaining berms. This action reduced the 
system to six larger cells, changed the aspect ratio from 
26:1 to 5.5:1, and increased water depth near the entry 


zone. Removing the three interior berms also opened up 
an additional 30,000 ft 2 in each of the remaining cells to 
serve as part of the wetland. 

The initial selection of long, narrow wetland channels 
was consistent with wetland design experience available 
in 1983-1984, but experience has since shown that proper 
treatment can be achieved in FWS wetlands with aspect 
ratios as low as 2:1 to 3:1 as long as the system is prop¬ 
erly constructed and the inlet and outlet structures allow 
for uniform flow through the system. 

Separate outlet structures at seven of the 11 lagoon cells 
allow the operator to select the lagoon cell(s) with the least 
algae for discharge to the wetland. This technique has been 
very effective at algae removal as long as sufficient plants 
and litter are present in the wetland, and as long as veloc¬ 
ity of flow in large open water zones is sufficient to prevent 
redevelopment and discharge of algae to the FWS sys¬ 
tem. 

Toxicity discharge limits imposed by the State of Califor¬ 
nia for un-ionized ammonia could not be met by the exist¬ 
ing pond-wetland system in the present mode of opera¬ 
tion. The existing point discharge is planned to be aban¬ 
doned and the effluent from the wetland component to be 
used for irrigation in a slow-rate land-treatment system. 

8.1.4 Ouray, Colorado 

8.1.4.1 Background 

Ouray is located in southwestern Colorado, about 60 
miles north of Durango, on State Route 50. Its population 
is about 2,500 in summer and about 900 in winter. The 
town is at an elevation of 7,580 ft in a mountain valley and 
experiences severe winter conditions. 

The free water surface (FWS) wetland at Ouray receives 
influent from a two-cell aerated lagoon and provides sec¬ 
ondary treatment prior to chlorine disinfection/dechlorina¬ 
tion and final discharge to the Uncompahgre River. The 
NPDES monthly average discharge limits are BOD 5 30 mg/ 
L, TSS 30 mg/L, and fecal coliforms 6000 CFU/100 mL. 
The wetland was designed for an expected winter water 
temperature of 3°C and a summer water temperature of 
20°C, with a 25% safety factor on sizing for BOD removal, 
based on existing design equations. 


138 











Table 8-5. 

Month 

Wetland Effluent Characteristics, Gustine, CA 

Temp. °F BOD 5 , mg/L 

TSS, mg/L 

NH 4 -N mg/L 

TKN, mg/L 

1996 

JAN 

48 

21 

20 

13 

15 

FEB 

54 

20 

22 

16 

18 

MAR 

55 

16 

24 

11 

16 

APR 

61 

30 

45 

4 

7 

MAY 

64 

29 

57 

3 

9 

JUN 

72 

18 

41 

2 

7 

JUL 

75 

22 

36 

7 

15 

AUG 

70 

47 

71 

- 

- 

SEP 

63 

32 

39 

- 

- 

OCT 

59 

26 

43 

- 

- 

NOV 

52 

28 

42 

- 

- 

DEC 

48 

24 

50 

- 

- 

1997 

JAN 

48 

25 

26 

- 

- 

FEB 

50 

22 

27 

- 

- 

MAR 

54 

27 

38 

- 

- 

APR 

61 

24 

34 

- 

- 

MAY 

66 

22 

26 

- 

- 

JUN 

70 

31 

34 

- 

- 


The design flow for the 2.2-acre wetland system is 0.250 
mgd in winter and 0.363 mgd in summer. As shown in Fig¬ 
ure 8-5, the wetland includes two parallel trains with three 
cells each. The two trains operate in parallel, and one can 
be taken out of service during the summer months for 
maintenance if required. The curved configuration of these 
wetland cells was selected in part because of the confined 
site and in part for aesthetic reasons. The water depth in 
the cells is adjustable via the outlet from a minimum of 8 in 
to a maximum of 18 in. This water level is increased to the 
maximum depth prior to the onset of winter to provide the 
maximum possible detention time during the low tempera¬ 
ture periods and to provide additional depth for ice forma¬ 
tion on the water surface during the winter months. 

A perforated manifold is used for both inlet and final out¬ 
let structures for the two sets of cells. Internal transfer from 
cell to cell is accomplished with two parallel pipes through 
each internal berm. The wetland cells are lined with 30-mil 
HDPE membrane liners to prevent seepage since the lo¬ 
cal soils are sandy clay loams. The detention time in the 
system depends on water depth and on the presence of 
winter ice. At minimum water depth the HRT is 2.2 d; at 
maximum water depth without ice, the HRT is 3.8 d. Lo¬ 
cally obtained cattails (Typha spp.) and bulrush (Scirpus 
spp.) were planted in the wetland cells. The vegetation is 
continuous, and there are no intended open water zones. 

The wetland was designed in 1992, constructed during 
the spring and summer of 1993, planted in October 1993, 
and placed in partial operation in November 1993. It has 
been in continuous operation since that time. 

The construction costs for the entire system, including 
aerated lagoons, chlorine disinfection/dechlorination equip¬ 
ment, and miscellaneous features was $816,530 (1993$). 
Construction costs for just the treatment wetlands is esti¬ 


mated to be about $108,500 or $49,300 per acre, or $300 
per 1000 gpd of design capacity. 

8.1.4.2 Financial Arrangements 

The construction costs for the Ouray system were funded 
by a state revolving-loan fund as administered by the State 
of Colorado in 1993. O&M costs for the system are funded 
with a surcharge on the consumer’s water bills. The total 
monthly O&M cost for the entire system at Ouray is $2,625; 
most of this is related to power costs, sludge removal from 
the aerated lagoon, and laboratory testing for NPDES 
monitoring. The average O&M costs for the wetland com¬ 
ponent is estimated to be about $200 per month for minor 
maintenance tasks. 

8.1.4.3 Construction and Start-up Procedures 

A geotechnical investigation was undertaken at the site 
for the new wetlands to determine underlying soil proper¬ 
ties and ground water conditions. Soil borings to several 
feet below the final wetland grade revealed the presence 
of sandy clay loams with sand and gravel inclusions, with 
an unconfined ground water aquifer at greater depth. These 
soils were typical of the local flood plain and were consid¬ 
ered too permeable, so a membrane liner was selected 
for the wetland. 

Clearing and grubbing was the first construction activity 
at the new wetland site. This was followed by grading, berm 
construction, and liner placement. Prior to liner placement, 
the subgrade was leveled and compacted to 90% of Proc¬ 
tor density to preserve the intended grade during subse¬ 
quent construction activities. After the liner was placed, 
1.5 ft of local sandy clay loam was placed in the wetland 
bottom to serve as the rooting medium for the wetland 
vegetation. The curved configuration of the wetland cells 
increased construction costs somewhat, but the site was 


139 















To Discharge 



Figure 8-5. Schematic diagram of the wetland system at Ouray, CO 


too confined to permit construction of a typical rectangular 
system with straight sides. 

Treatment wetland cells were hand planted by correc¬ 
tional facility inmates with locally obtained bulrush and 
cattail plants. The vegetation was planted on about 18-in 
centers; at this density about 43,000 plants were required. 
The bed was flooded with about 8-in of water and main¬ 
tained in that condition until sufficient new plant growth 
was observed. Some wastewater was applied during the 
remainder of the 1993 winter, but full-scale operation did 
not commence until the spring of 1994. 

This system experiences subfreezing air temperatures 
for extended periods each winter. An ice cover at least 6- 
in thick persists for at least six months. 

The inlet and outlet devices for each set of cells are 8-in, 
perforated, Schedule 80 PVC pipe. These pipes were laid 
in a 2-ft-wide, 18-in-deep trench extending the full width of 
the cell. The trench bottom and sides are protected with 2- 
to 4-in riprap. One end of each manifold has a 90% elbow 
and a capped riser extending above the water surface to 
serve as a cleanout if required. The effluent manifolds con¬ 
nect to a concrete outlet structure that contains adjustable 
outlet riser pipes for controlling the water level in the cells. 

There are no special O&M requirements for these wet¬ 
land units, including harvesting or other plant management 
procedures. Raising and lowering the wetland water lev¬ 
els on a seasonal basis and sampling for NPDES compli¬ 
ance are about the only O&M tasks required. There have 
been no problems with muskrats or other animals damag¬ 


From Lagoon 


ing the plants as has occurred at several other wetland 
systems. The wetland tasks listed in the O&M manual in¬ 
clude weekly cleaning of effluent debris screens, weekly 
checking of berms for erosion or muskrat damage, clean¬ 
ing influent and effluent manifolds as required, and occa¬ 
sional muskrat control. Mosquitoes have not been a prob¬ 
lem at this site. 

8.1.4.4 Performance History 

It is typical for most small systems, including the Ouray 
system, to monitor only for NPDES limits, and for that rea¬ 
son to sample only the untreated (raw) wastewater and 
the final effluent. As a result, the actual influent to the wet¬ 
land component is not known. Data from the Ouray sys¬ 
tem for the 1995-1996 period is shown in Table 8-6. 

Based on limited data, the aerated lagoon at Ouray is 
estimated to remove about 54% of influent BOD 5 and 65% 
of influent TSS. On that basis, with the average wetland 
influent in 1995 at 58 mg/L BOD 5 and 63 mg/LTSS, the 
wetland achieved an average removal of 83% BOD 5 and 
90% TSS. In 1996, the average wetland removal percent¬ 
ages were 88% for BOD 5 and 91 % for TSS. Wetland aver¬ 
age effluent fecal coliform concentrations during 1995 and 
1996 were 570 CFU/100 mL and 1300 CFU/100 mL, re¬ 
spectively. All of the monthly values were well below the 
NPDES limit of 6000 CFU/100 mL, so it was not neces¬ 
sary to operate the disinfection/dechlorination equipment 
installed at the site. 

8.1.4.5 Lessons Learned 

• The Ouray system incorporated many improvements 
learned from earlier FWS systems, including perforated 


140 





















Table 8-6. BOD & TSS Removal for Ouray, CO 


Date 

BOD In, 
mg/L* 

BOD Out, 
mg/L“ 

TSS In, 
mg/L* 

TSS Out, 
mg/L ** 

1995 

Jan 

84 

5 

124 

11 

Feb 

78 

8 

122 

8 

Mar 

84 

7 

216 

10 

Apr 

132 

9 

182 

5 

May 

66 

8 

152 

11 

Jun 

174 

15 

196 

4 

Jul 

180 

14 

170 

5 

Aug 

216 

10 

316 

5 

Sep 

204 

18 

296 

4 

Oct 

132 

16 

144 

6 

Nov 

96 

6 

146 

5 

Dec 

78 

3 

98 

4 

Average 

127 

10 

180 

6 

1996 

Jan 

90 

4 

176 

4 

Feb 

92 

6 

154 

6 

Mar 

95 

2 

184 

2 

Apr 

60 

5 

178 

2 

May 

78 

13 

68 

11 

Jun 

96 

11 

109 

6 

Jul 

162 

7 

334 

9 

Aug 

168 

11 

226 

5 

Sep 

120 

5 

102 

4 

Oct 

126 

8 

127 

5 

Nov 

108 

3 

121 

3 

Dec 

78 

2 

160 

4 

Average 

106 

6 

162 

5 


‘Untreated wastewater 
“Final system (wetland) effluent 


manifolds extending the full width of the wetland cells 
for inlets and outlets, cleanouts on the ends of these 
manifolds, and a simple adjustable outlet structure for 
control of the water level in the wetland cells. 

• The adjustable outlet structure for water-level control 
was essential for the water level to be raised during 
the winter months to accommodate expected ice for¬ 
mation. 

• Large-sized riprap (4- to 6-in size) as a permanent slop¬ 
ing cover for both the influent and effluent manifolds 
excludes clogging debris and prevents algae devel¬ 
opment. This technique precludes periodic cleaning 
of a screen over the effluent manifold that would have 
been installed to prevent accumulation of debris. 

• Bats and dragonflies contribute to mosquito control dur¬ 
ing the warm summer months, so mosquitoes and simi¬ 
lar insect vectors have not been a health problem at 
this system. 

• Odors occasionally noticed at the inlet end of the wet¬ 
land cells are caused by accumulation of TSS and al¬ 
gae carried over from the final aerated lagoon cell 
because the settling zone in this final lagoon cell is too 
small to be completely effective. 

• Ice cover and snow accumulation have provided ac¬ 
ceptable thermal protection for the FWS system, and 


the system has not needed alteration during the win¬ 
ter months. In response to State of Colorado concerns 
that FWS wetlands would not sustain acceptable per¬ 
formance during low-temperature winter months, the 
lagoon aeration system had been designed to allow 
longer operational periods during winter months to 
provide additional treatment so the wetland cells could 
have been bypassed during winter months, if neces¬ 
sary. 

• Chlorination/dechlorination equipment included in the 
original design at the insistence of the State of Colo¬ 
rado has not been used, as the wetland effluent has 
been consistently below permit limits. 

8.2 Vegetated Submerged Bed (VSB) 
Systems 

8.2.1 Village of Minoa, New York 

8.2.1.1 Background 

The Village of Minoa is a small residential community of 
approximately 3,700 in central New York state east of Syra¬ 
cuse. The average daily flow to the wastewater treatment 
plant in 1993 was approximately 0.35 mgd, but peak flows 
as high as 1.6 mgd had been recorded. Efforts between 
1990 and 1993 to abate the high rates of infiltration and 
inflow were unsuccessful, and the Village of Minoa was 
forced into a consent order with the New York State De¬ 
partment of Environmental Conservation (NYSDEC) to 
correct discharge violations. 

In 1994 the village decided to use a VSB constructed 
wetland system to treat primary effluent to secondary ef¬ 
fluent standards, with an ultimate oxygen demand limit that 
required at least partial nitrification. The VSB system also 
would be used during wet weather conditions to treat 
640,000 gpd of wet weather flow. The dry weather capac¬ 
ity of the VSB system was to be 160,000 gpd, but the ac¬ 
tual constructed size of the system was smaller than the 
original design, reducing the design capacity to approxi¬ 
mately 130,000 gpd. The treatment goal also was changed 
from a BOD 5 concentration of less than 30 mg/L and par¬ 
tial nitrification to BOD 5 alone. 

Two New York state agencies and the U.S. EPA pro¬ 
vided grant funds to the village for incorporation of several 
special features in the VSB system and for a research and 
technology transfer study of the system by researchers at 
Clarkson University, Potsdam, NY. 

The VSB system consists of three cells that can be op¬ 
erated in parallel, combined parallel and series, or series 
modes. Cells 1 and 2 are approximately the same size 
(0.17 ha or 0.42 acres). Cell 3 is significantly smaller and 
is irregularly shaped (0.1 ha or 0.25 acres) (Figure 8-6). At 
the inlet end, the media depth is 0.5 m and the bottom 
surface has a slope of 1%, resulting in a bed depth of ap¬ 
proximately 0.9 m at the outlet end and an average depth 
of 0.76 m. The upper 7.6 cm of the beds consist of 0.6 mm 
pea gravel, which allowed for the establishment of wet- 


141 









a) Period One - Continuous Flow in Parallel 



0.17 ha 0.175 ha 0.10 ha 


P = Phragmites 
S = Scirpus 
N = No Plants 


b) Period Two - Fill and Drain Flow in Series 



c) Period Three - Combination Sequence (fill and draw [1 and 2] to continuous 3) 



Figure 8-6. Schematic of Minoa, NY, VSB system 

land plants. The larger treatment media have an effective 
size of approximately 1.9 cm and a measured porosity of 
0.39. The cells are lined with a 60-mil HDPE liner. 

Each cell is divided in half longitudinally by an extension 
of the liner to the top of the media. Three of the half cells 
were planted with Phragmites, two of the half cells were 
planted with Scirpus, and the final half cell was left 
unplanted. This planting scheme allowed for performance 
comparisons of planted versus unplanted cells and Scirpus 
versus Phragmites. The system is depicted in Figure 8-6. 

In addition to the multicell design and multiple opera¬ 
tional modes, the VSB system at Minoa incorporated sev¬ 
eral other special features, including trilevel observation 


well clusters within each half cell, specially designed inlet 
weirs, thermistors beneath one of the cell liners and at 
various levels within the cell, a dual-level effluent with¬ 
drawal, and an adjustable water-level control. 

The specific goals of the research/technology transfer 
efforts were the following: 

1. Establish optimum hydraulic, organic, and solids 
application rates necessary to achieve Village of 
Minoa NPDES permit limitations. 

1 

2. Conduct testing to determine the impact of wet-event 
peak-day hydraulic impacts on treatment perfor¬ 
mance. One of the project objectives was to evalu- 


142 


























ate the performance of the system under the maxi¬ 
mum hydraulic design condition of 640,000 gpd. 

3. Conduct tracer dispersion testing to measure actual 
bed HRT and “in-place” hydraulic conductivities, to 
evaluate impacts due to clogging, and to determine 
the extent of short-circuiting. 

4. Correlate ambient and wastewater temperature data 
with observed removal efficiency for BOD, UOD (Ul¬ 
timate Oxygen Demand), and ammonia nitrogen. 

5. Evaluate the effect of plants (vs. no plants) and spe¬ 
cific plants (Scirpus vs. Phragmites) on treatment 
performance. 

6. Evaluate the effect of vegetative harvesting on nu¬ 
trient removal efficiency. 

7. Provide data for calibrating an existing VSB heat- 
loss model that predicts the substrate temperature 
at various locations in the system. 

8. Evaluate effects of series- versus parallel-flow con¬ 
figurations on treatment performance. 

9. Conduct a detailed energy audit to establish the 
energy benefits of this system in comparison with a 
conventional treatment approach. 

Construction costs for the system are summarized in 
Table 8-7. It should be noted that (1) the work at Minoa 
was completed under adverse weather conditions and a 
tight construction schedule because of the consent order 
requirements, and (2) costs reflect all of the special fea¬ 
tures incorporated in the system for research. 

8.2.1.2 Financial Arrangements 

The costs of the Minoa wetland system associated with 
the research aspects of the project were funded by the 
U.S. EPA and the two State of New York agencies. The 
remaining capital costs of the project were funded with a 
state revolving-fund loan under the innovative and alter¬ 
native system program. 

8.2.1.3 Construction and Start-up Procedures 

As noted previously, the work at Minoa was completed 
under adverse weather conditions and a tight construction 
schedule because of the consent order requirements. Dur¬ 
ing the establishment of the wetland plants throughout most 


Table 8-7. Village of Minoa VSB Construction Costs (Fall, 1994) 


Sitework 

$135,500 

60 Mil HDPE Liner 

82,500 

Wetland Media 

104,500 

Wetland Plants 

29,000 

Piping & Distribution 

179,000 

Miscellaneous 

25,500 

Total 

$568,000 


of 1995, the wetland cells received only secondary efflu¬ 
ent from the existing trickling filter. 

8.2.1.4 Performance History 

The performance of the Minoa VSB system in treating 
primary effluent can be divided into three periods. During 
the first period of January 1996 to March 1997, the system 
was operated as a conventional VSB system, with the three 
cells in parallel. From April 1997 to March 1998, the three 
cells were operated in series and in a sequential fill-and- 
drain mode. From March 1998 to the writing of this manual, 
the system has been operated in a different fill-and-drain 
mode. Two cells, cells 1 and 2, operate in parallel but in 
alternating fill-and-drain mode, similar to sequencing batch 
reactors. The third cell, cell 3, operates in series-flow, but 
with a constant water level, following the other two cells. 

Conventional Parallel Operation 

The BOD t removal performance of the Minoa VSB sys¬ 
tem in the conventional mode was very poor when com¬ 
pared with the original design expectations. The three cells 
were operated in parallel flow, but with different HRTs. The 
performance of the Minoa system in BOD 5 removal during 
the first 10 months of conventional operation is summa¬ 
rized in several of the figures (identified as CU) in Chapter 
5 and can be compared with two other systems. The false 
performance expectations for the system were based on a 
design equation developed with limited data, mostly from 
VSB systems treating lagoon and pond effluents. The equa¬ 
tion assumed that BOD 5 removal performance is depen¬ 
dent on temperature. Pollutant removal was not found to 
vary significantly with temperature at Minoa. 

The performance of the Minoa VSB system in TSS, TKN, 
and total phosphorus removal during this period was simi¬ 
lar to the performance of other VSB systems treating sep¬ 
tic tank effluents (see Chapter 5 figures). TSS and BOD 
removal were reasonably good, whereas TKN and total 
phosphorus removal was quite poor. 

Tracer study results from Minoa were also very similar 
to tracer study results from other VSB systems. After one 
year of operation, a significant fraction of the wastewater 
flowed under the shallow root zone of the system. Also 
observed were substantial dead volumes and typical 
amounts of dispersion within the media. 

Comparing the treatment performance of planted and 
unplanted half cells, the Clarkson researchers found that 
the unplanted half-cell performance was equal to the 
planted cells for all pollutants measured. They also found 
that the Phragmites cells removed more COD, TKN, and 
total phosphorus than the cells planted with Scirpus. 

Series-Flow, Sequential Fill-and-Drain Operation 

The three cells of the Minoa VSB system were operated 
in series-flow, sequential fill-and-drain operation for approxi¬ 
mately 12 months. The operation during this time made 
use of the dual effluent piping to achieve the fill-and-drain 


143 














operation, even though the flow through all three cells was 
continuous. The water surface in a cell was controlled by 
opening and closing the bottom drain line valve. When the 
drain valve was closed, effluent from a cell flowed through 
the upper effluent piping. 

A typical cycle started with wastewater flowing through 
a filled cell 1. The drain valve in cell 1 was opened 
while drain valves in the other cells 2 and 3 were 
closed. Twenty-four hours later, the drain valve in 
cell 1 was closed and the drain valve in cell 2 was 
opened. After 24 hours in this configuration, the drain 
valve in cell 2 was closed and the drain valve in cell 
3 was opened. It should be noted that this mode of 
operation was possible at Minoa because of the sig¬ 
nificant drop in elevation from cell 1 to Cell 3. At a 
flow rate of 130,000 gpd, the draining of cells 1 and 
2 from their upper levels would take four to five hours, 
while cell 3 required only three hours. In filling, cells 
1 and 2 would require 24 hours, while cell 3 required 
12 hours. 

The performance in BOD 5 removal during the sequen¬ 
tial fill-and-drain operation was significantly better than 
during the previous period of conventional operation. Ef¬ 
fluent BOD 5 averaged less than 15 mg/L while the system 
was treating a much higher flow, and performance improved 
during the latter months of the period. TSS removal was 
also good, but TKN and total phosphorus removal did not 
improve significantly. One of the most important improve¬ 
ments in the operation of the Minoa system during this 
period was the reduction in the hydrogen sulfide odors that 
had plagued the system during the period of conventional 
operation. 

Alternating Parallel Fill-and-Drain/Series-Flow Operation 

Operation since March 1997 has had cells 1 and 2 oper¬ 
ating in an alternating fill-and-drain mode followed by cell 
3 operating in a constant-saturated mode. The pollutant 
removal performance for BOD 5 and TSS has remained 
quite good, and there has been a significant increase in 
nitrogen removal performance. 

8.2.1.5 Lessons Learned 

• Fill-and-drain operation can significantly increase the 
BOD and nitrogen removal performance of conven¬ 
tional VSB systems. 

• BOD 5 removal is not temperature dependent in con¬ 
ventional VSB systems. 

• Because of the potential for severe odor problems, con¬ 
ventional VSB systems must be designed to have lower 
organic loading rates when sited near households. 

8.2.2 Mesquite, Nevada 

8.2.2.1 Background 

Mesquite, Nevada, is located on 1-15 near the Nevada- 
Arizona border, about 112 miles east of Las Vegas. The 


original treatment system for the community included 
coarse screening and aerated facultative ponds followed 
by storage ponds and land application on 62 acres of al¬ 
falfa fields. The State of Nevada required an effluent with 
BOD at 30 mg/L and TSS at 90 mg/L prior to land applica¬ 
tion. The effluent at the Mesquite facility often exceeded 
these limits, so an upgrade was required. 

A 1989 facility plan for the upgraded facility recom¬ 
mended an increase in total treatment capacity to 1.2 mgd, 
additional aerated lagoons with lagoon effluent to either 
overland flow terraces or a VSB, and either of these fol¬ 
lowed by rapid infiltration basins. The VSB concept was 
selected for this system because a free water surface 
(FWS) wetland would have required a larger land area, 
might not have been as effective for algae removal, and 
would have been more susceptible to mosquito problems. 
The design flow to the VSB was 400,000 gpd, with the 
remainder routed from the lagoon to the overland flow 
slopes. The existing facultative pond contained multiple 
cells, and three of these were selected for conversion to 
VSBs. The total VSB area was 4.7 acres. 

The modified aerated lagoons were expected to produce 
an effluent with about 70 mg/L, and the VSB wetlands were 
designed to produce an effluent with 30 mg/L BOD 5 in the 
coldest month, which was January. The design model used 
for BOD 5 removal is temperature dependent, so the sys¬ 
tem was sized to produce the target effluent value during 
the coldest month. There were no NPDES discharge lim¬ 
its for the VSBs since they were designed to discharge to 
rapid infiltration basins and not to a receiving stream. 

A schematic plan is shown in Figure 8-7 for one of the 
three similarly configured VSB units. Each of the three 
parallel units contained four parallel cells as shown on the 
figure. The flow path in each of the four cells averaged 50 
ft, and the cell width averaged 380 ft. This configuration 
produces an average aspect ratio (L:W) of 0.13:1. This 
very low aspect ratio was selected following observation 
of surface flooding and related problems with VSB sys¬ 
tems in Louisiana, Mississippi, and Oklahoma that had 
aspect ratios of 10:1 or more and no provision for the nec¬ 
essary hydraulic gradient to overcome the frictional resis¬ 
tance of a very long flow path. In addition to the short flow 
path distance provided at Mesquite, a bottom slope of 1% 
was provided for the cell bottoms. 

8.2.2.2 Financial Arrangements 

Funding for construction of this new system was pro¬ 
vided by a combination of municipal bonds and the State 
of Nevada’s revolving-loan fund. The total construction 
costs for the VSB component at Mesquite was $515,000 
(1990$), or $109,600 per acre, or $1,287 per 1000 gallons 
of treatment capacity. The area cost is less than the 
$178,000/acre (1990$) at the comparable VSB system in 
Mandeville, Louisiana (see Mandeville case study), and 
the difference is probably due to the higher cost of rock 
and gravel in Louisiana. Land and liner costs for the Mes¬ 
quite project were zero because existing lagoon cells were 


144 






< 


380 ft 




Figure 8-7. Schematic diagram of typical VSB (one of three) at Mesquite, NV 


converted to VSB units. The O&M costs are funded di¬ 
rectly by a sewer charge for each connected user; a single¬ 
family connection would pay approximately $8.63 per 
month for this service. 

8.2.2.3 Construction and Start-up Procedures 

Construction of the new system components was com¬ 
pleted in late 1990 and start-up occurred in April 1991. 
The original lagoon cells were lined with asphaltic con¬ 
crete. These were prepared for the new VSB units by drain¬ 
ing and drying, and then placement and compaction of lo¬ 
cal clay soil backfill to a depth of about 2 ft. This backfill 
was then graded to provide the desired 1% slope for the 
bottom. This 2-ft of compacted soil also ensured the im¬ 
permeability of the bottoms. The effluent manifolds were 
placed and leveled on the bottom prior to gravel bed con¬ 
struction. Gravel for the bed was transported from the lo¬ 
cal pit, dumped in the wetland cell, and spread with a small 
bulldozer. Trenches for the coarse inlet zone rock were 
excavated and backfilled after placement of the entire 32- 
in-deep gravel layer. The 2-in layer of fine gravel/coarse 
sand was then placed on the surface of the bed, with the 
exception of the inlet and outlet zones. Posts were then 
driven into the gravel layer for support of the distribution 
manifold pipes. Construction of external piping, outlet struc¬ 
tures, and pumping stations then completed the work. 

Flow distribution to the three VSB units utilized orifice 
plates to split the flow, and 8-in perforated pipe manifolds 
were used in each cell for both distribution and effluent 
collection, as shown in Figure 8-7. The influent pipes were 
elevated slightly above the bed surface, and the effluent 


manifolds were at the bottom of the bed. The main VSB 
bed consisted of a 32-in depth of washed river gravel rang¬ 
ing in size from 0.4 in to 1.0 in obtained at a local gravel 
pit. An inlet zone underneath each inlet pipe contains 2-in 
to 4-in rock to ensure rapid infiltration and distribution. This 
zone is about 3 ft wide at the top and extends the full 
depth of the bed. The gravel in the main bed was then 
covered with about a 2-in layer of fine gravel/coarse sand 
mixture to aid in the germination and growth of the veg¬ 
etation. 

There were no soils or geotechnical investigations at 
this site since existing lined lagoon cells were to be used 
for the new VSBs. The only geotechnical activity involved 
with this project was to find a suitable source for the rock 
and gravel required. The layer of fine gravel/coarse sand 
was chosen because the intended method of planting was 
hydroseeding. A layer of fine gravel/coarse sand mixture 
was placed on top of the gravel in the VSB cells to serve 
as a growth substrate for the intended hydroseeding. The 
first bed was hydroseeded in July 1991 at a rate of 25 lb/ 
acre of seed mixed with 2500 Ib/acre of mulch fiber. Sprin¬ 
klers were then used to periodically flood the surface of 
the bed to encourage germination and growth. By Sep¬ 
tember 1991, only 20% germination could be observed. 
Alkali bulrush (Scirpus robustus) was selected as the sole 
vegetation type for all of the VSB cells. Again hydroseeding 
was attempted but proved not to be successful. Planting 
by hand with locally available plant materials (from ditches, 
etc.) was successfully completed during the second year 
of system operation. In 1997, the VSB cells were com¬ 
pletely covered with healthy vegetation. There is no har- 


145 



























vesting or other vegetation- management procedures at 
this site. 

Water-level control in two of the three VSB units is pro¬ 
vided by overflow weirs in the outlet structures. In the third 
unit, water-level control depends on float-switch settings 
for the discharge pump. In addition, the piping and distri¬ 
bution and collection system were designed to operate with 
a continuous 0.4 mgd recycle flow (100% of forward flow). 

As of August 1997, an additional plant expansion was 
underway at the Mesquite system. The city is growing rap¬ 
idly as a retirement/recreational community and a number 
of golf courses are under construction or planned. To pro¬ 
vide irrigation water for these golf courses, the wastewa¬ 
ter plant expansion is including an oxidation ditch with ni¬ 
trification/denitrification capability and UV disinfection to 
meet the necessary bacterial limits for golf course irriga¬ 
tion. The VSB/overland flow/rapid-infiltration units at Mes¬ 
quite will remain in stand-by use and will be operational 
during high-flow winter months. 

8.2.2.4 Performance History 

The inlet orifice plates were provided to split the influent 
flow proportionally to the surface area of each VSB unit 
since there were slight variations in the size of the three 
units. Table 8-8 presents average VSB performance data 
during the period June 1992 through May 1993. Data are 
not available on the performance of individual units or cells 
within a unit. 

Although the system met its effluent BOD target on an 
annual average basis, there were monthly variations, as 
shown in Table 8-9. However, these excursions had mini¬ 
mal impact on the rapid-infiltration system. 

These 1992-1993 performance results were achieved 
without recycling VSB effluent. However, at the time of the 
1997 site visit, recycle at 400,000 gpd was practiced con¬ 
tinuously and produced essentially the same performance 
results shown in the tables. Recycle was only considered 
to be essential in the very hot and dry summer months in 
order to keep the plants on the beds alive and functional. 

In the general case, algae forms in the lagoon and is 
separated in the VSB, and the decomposition of the algae 
releases additional ammonia and organics. As a result, 
the effluent ammonia and organics are elevated due to 
internal loading during the warmest periods. Removal dur¬ 
ing the warmer months of the year is believed to be offset 


Table 8-8. Summary Performance, Mesquite, Nevada, VSB Compo¬ 
nent, June 1992-May 1993 


Parameter 

Influent, mg/L 

Effluent, mg/L 

% Removal 

bod 5 

64 

29 

55 

TSS 

57 

13 

77 

nh 4 -n 

16 

10 

38 

TKN 

29 

16 

46 

TP 

7.4 

6.2 

16 


Table 8-9. Effluent Characteristics, Mesquite, NV, VSB Component, 
June 1992-May 1993 


Month 

Temp. 

°C 

BOD 

mg/L 

TSS 

mg/L 

nh 4 -n 

mg/L 

TKN 

mg/L 

TP 

mg/L 

1992 

Jun 

21.6 

32 

6 

3.3 

6.7 

5.0 

Jul 

26.7 

24 

6 

4.3 

6.4 

5.3 

Aug 

27.1 

26 

6 

4.5 

7.6 

4.8 

Sep 

23.6 

22 

5 

4.1 

6.8 

5.5 

Oct 

19.1 

37 

5 

3.3 

5.6 

6.1 

Nov 

13.5 

32 

22 

5.3 

8.6 

5.8 

Dec 

7.5 

27 

16 

15.7 

22.3 

4.7 

1993 

Jan 

8.1 

24 

14 

19.8 

29.7 

6.1 

Feb 

12.7 

24 

18 

21.9 

29.9 

8.0 

Mar 

13.9 

23 

16 

22.1 

29.9 

9.2 

Apr 

16.2 

49 

17 

12.4 

23.6 

7.1 

May 

20.3 

27 

21 

6.0 

9.5 

7.0 


by plant uptake during the growing season. Subsequent 
data would be very useful to help identify the annual cycle 
over several years. 

8.2.2.5 Lessons Learned 

• The wetland configuration and cross section shown in 
Figure 8-7 were designed to maximize the available 
area in the former lagoon cell, while at the same time 
minimizing the aspect ratio. 

• Surface overflows are due to improper hydraulic de¬ 
sign rather than clogging. 

• Subdividing each VSB into four separate cells with the 
right slope in each cell to ensure proper flows required 
very careful grading of subgrade soils that significantly 
increased the cost and complexity of construction. 

• Subdividing each unit into two cells by applying influ¬ 
ent along the centerline and collecting effluent along 
the two sides would have produced an aspect ratio of 
0.26:1. 

• Converting each former lagoon cell to a single wet¬ 
land bed, with application along one long side and ef¬ 
fluent collection along the opposite side, would have 
produced an aspect ratio of 0.5:1, with a level subgrade 
and the water level and hydraulic gradient controlled 
by an adjustable outlet. 

• Continuously flooding the bed with a few inches of wa¬ 
ter after hydroseeding, rather than intermittently wet¬ 
ting it, may have improved germination, as would plant¬ 
ing in a more moderate season in the desert climate. 

• Hand planting of shoots or rhizomes in the gravel of a 
VSB system is preferred. Potted shoots and rhizome 
material for a wide variety of plant species are com¬ 
mercially available. 

• An effluent recycle feature permitting 100% recycle is 
not typical at most VSB systems and was not neces¬ 
sary for water quality purposes. 


146 









• Routine maintenance requirements at this system are 
minimal and consist of periodic pump inspections and 
monthly cleaning of orifices in the influent distribution 
manifolds. 

8.2.3 Mandeville, Louisiana 

8.2.3.1 Background 

Mandeville, Louisiana, is located on the northern shore 
of Lake Pontchartrain at the end of the causeway bridge 
from New Orleans. The 1997 population of Mandeville was 
about 10,000, and the suburban residential community was 
expanding rapidly. A vegetative submerged bed (VSB) was 
selected as a component in the new wastewater treatment 
facilities at the recommendation of the State of Louisiana 
and the U.S. EPA Region VI. The system was constructed 
during 1989 and placed in operation in February 1990, with 
a design flow of 1.5 mgd. The system discharges to Bayou 
Chinchuba, which drains to Lake Pontchartrain. The 
NPDES limits are BOD 5 10 mg/L, TSS 15 mg/L, NH3/NH4 
5 mg/L, fecal coliform 200/100 mL, and a maximum pH of 
9. 

The new system was constructed at the site of the 
community’s original three-cell facultative lagoon, and one 
of the original cells was retained for temporary treatment 
and later abandoned at the completion of the new system. 
A second original cell was deepened and converted to a 
partial-mix aerated lagoon with three cells operated in se¬ 


ries and submerged perforated tubing in the first two cells. 
The hydraulic residence time (HRT) in this new lagoon was 
about 15 days at design flow. The third original lagoon cell 
was converted to a three-cell VSB gravel bed. The VSB 
cells operate in parallel. Other new elements in the sys¬ 
tem included a headworks containing a bar screen and 
grit chamber, final disinfection with UV, and an effluent 
pumping station. All of these major system components 
are shown in Figure 8-8. 

At the time this system was designed, sizing criteria were 
5 acres per mgd of design flow, a one- to two- day HRT in 
the VSB, and an aspect ratio (L:W) of at least 10:1 to en¬ 
sure plug flow conditions. These criteria assumed that the 
VSB influent would contain about 30 mg/L of BOD 5 and 
TSS following treatment in the aerated lagoon. All of these 
criteria were applied at Mandeville except the 10:1 aspect 
ratio, which could not be used due to the preexisting con¬ 
figuration of the facultative lagoon cell. The average as¬ 
pect ratio of the three VSBs is about 2.5:1. 

The three VSBs are separated by low internal earthen 
berms that provide about 1.5 ft of freeboard above the 
gravel surface in the bed. The external berms are the pre¬ 
existing dikes of the former facultative lagoon. The bottom 
surface area is 6 acres. The VSB bed is composed of a 
1.5-ft depth of crushed limestone rock (2- to 4-in size) over- 
lain by 6 in of granite gravel (0.5- to 1-in size). The surface 
layer of gravel was considered necessary as a rooting 



Figure 8-8. Schematic of VSB system at Mandeville, LA 


147 






































medium for the vegetation. Softstem bulrush (Scirpus 
validus) was selected as the sole vegetation type, and nurs¬ 
ery-grown shoots were planted on about 4-ft centers. An 
annual harvest of these plants was recommended by the 
designers and was practiced for several years after start¬ 
up. ' 

A 20-in PVC pipe conveys lagoon effluent to the VSB. 
This pipe connects to a PVC manifold extending the full 
width of the three cells. At three equidistant points in each 
cell, the manifold discharges to a 10-in outlet pipe that is 
valved and extends 25 ft into the bed. These outlet pipes 
are at the surface of the bed, and they each end in a 90° 
“down” elbow that penetrates into the rock layer. These 
nine gate valves were intended for flow control so that a 
cell could be taken out of service and/or flow could be ad¬ 
justed as required to produce a relatively uniform distribu¬ 
tion of flow. 

The effluent manifold for cells 1 and 3 is 21-in PVC and 
24-in PVC for cell 2. These manifold pipes were buried 
with the top of the pipe flush with the top of the coarse rock 
layer. Four-in-diameter holes were drilled on 8-in centers 
at the top center of these manifold pipes. These manifolds 
connect to the UV disinfection chamber, which then dis¬ 
charges to the sump of the discharge pump. The top of the 
gravel layer was graded level, as was the bottom of the 
bed, and no adjustment was possible in the water level in 
the bed, nor was it possible to drain the cells. 

A special feature in all three wetland cells is the inclu¬ 
sion of buried 6-in perforated PVC pipes. Two of these 
open-ended pipes are buried in each cell, about 6 in above 
the bed bottom in the coarse rock media. Their apparent 
purpose is redistribution of subsurface flow in case the entry 
zone of the bed becomes clogged with solids. Each pipe 
is 100 ft in length and is laid parallel to the flow direction; 
the two pipes in each bed are located about 35 ft on each 
side of the longitudinal bed centerline. 

The construction costs for the entire system, including 
the aerated lagoons, was about $3,000,000 (1990$). The 
cost for the VSB cells was about $590,000 (1990$), with 
about 70% of that for procurement and placement of the 
rock media and gravel layer. The materials used at 
Mandeville were barged from Arkansas, since rock and 
gravel are not readily available in this part of Louisiana. 
Other VSB projects in the vicinity have used rock and gravel 
barged from Mexico. The VSBs are not lined since the 
subsoils are clay and sandy clay. Since the exterior dikes 
for the former lagoon were utilized, construction costs were 
minimal (except for the cost of rock and gravel). Land costs 
were zero since the preexisting lagoon was municipally 
owned. The construction costs for this VSB were about 
$590 (1990$) per 1000 gallons of design flow, or $105,400 
per acre of treatment area for the 5.6-acre system. 

8.2.3.2 Financial Arrangements 

The construction costs for the Mandeville system were 
funded privately through bonds issued by the City of 


Mandeville. No grant or funding support was provided by 
the State of Louisiana or the U.S. EPA. The apparent rea¬ 
son was the relatively low position of the city on the grant 
priority list. The city, faced with the choice of curtailing com¬ 
munity growth or funding a new system itself, chose the 
latter option. The O&M costs for the system have been 
obtained as a surcharge on the consumer’s water bill. 

8.2.3.3 Construction and Start-up Procedures 

Construction activities commenced with draining of the 
existing facultative lagoon. The bottom was allowed to dry, 
and then accumulated sludge was removed and disposed 
of. The bottom was then leveled in preparation for backfill¬ 
ing with gravel. The concrete structures containing the UV 
disinfection components and the effluent pump station were 
also constructed at this time. The low interior earthen berms 
were then constructed to divide the lagoon cell into three 
parallel units. These interior berms permit foot traffic only. 
The rock and gravel were hauled by truck from the barge 
dock on Lake Pontchartrain to the site, dumped into the 
bed, and spread with small bulldozers. The entire coarse 
rock layer was placed and leveled before any gravel was 
placed as the top layer. The inlet and outlet manifolds were 
then installed and connected and rock backfilled around 
them (the top gravel layer was not placed in these inlet 
and outlet zones). The bed was then filled with water (with 
effluent from the temporary lagoon) to the top of the coarse 
rock. The bulrush shoots, obtained from a nursery in Mis¬ 
sissippi, were planted by hand on 4-ft centers, with their 
roots in contact with the water at the top of the coarse 
gravel. About 15,000 plants were planted in the three cells. 

Start-up of this system commenced immediately upon 
completion of construction. In some systems of this type, 
clean water is used to initially fill the bed, and the plant 
shoots are allowed to grow for four to six weeks prior to 
introduction of wastewater. In this case, lagoon effluent 
was introduced during the planting stage, and daily flow 
through the VSB commenced as soon as the aerated la¬ 
goons were operational. There were no special start-up 
procedures used at this site. However, a unique mainte¬ 
nance procedure was adopted for several years, which 
started with harvesting of weeds to encourage growth and 
spread of the bulrush, which evolved into a complete an¬ 
nual harvest of all vegetation and the removal and dis¬ 
posal of the harvested material. That practice has now been 
terminated. 

8.2.3.4 Performance History 

In 1991, the Mandeville system was selected by the U.S. 
EPA for a detailed eight-week performance evaluation. This 
effort included independent flow metering of system influ¬ 
ent, VSB influent and effluent, tracer studies to verify HRT, 
and weekly composite sampling and testing for BOD (total 
and soluble), COD (total and soluble), TSS, VSS, TKN, 
NH 4 -N, N0 3 , TP, DO, pH, and temperature. The study pe¬ 
riod commenced in mid-June 1991 and was completed by 
late September 1991. The average flow rate during this 
period was 1.16 mgd, indicating that 77% of the system 
design capacity was achieved in the second year of op- 


148 



eration. This is a reflection of the very rapid growth and 
residential construction in the community. A summary of 
the water quality performance data is given in Table 8-10. 
The tracer study, conducted only in cells 2 and 3 because 
the valves for cell 1 had been inadvertently closed, mea¬ 
sured a flow rate of 1.352 mgd, which indicated an actual 
HRT of 17.8 hours. This compared favorably with the theo¬ 
retical HRT of 18 hours for the same flow rate, assuming a 
porosity of 42% in the rock/gravel bed. At the time of the 
tracer test, surface water was apparent on portions of the 
wetland cells, but the majority of the flow was subsurface. 
If cell 1 had been operational during the test, it is believed 
that the actual HRT would have been close to the one-day 
theoretical HRT for the full system. 

Table 8-11 presents a summary of system performance 
data collected in 1996 and 1997. The values shown are 
the averages for the month shown. 

As shown in Table 8-11, the current actual flow exceeds 
the original design rate of 1.5 mgd, but the system contin¬ 
ues to meet the discharge limits for BOD and TSS but ex¬ 
ceeds the ammonia limit on a seasonal basis (i.e., non- 
compliance in the colder months). The routine compliance 
with BOD 5 and TSS limits is in part due to the reliability of 
these systems for removal of these parameters, but is also 
in part due to significant modifications to the system made 
in 1992. The present system configuration, with the sur¬ 
face aerators, the subsurface aerators, and the baffle cur¬ 
tains as shown in Figure 8-8, has been in place since 1992. 
The lagoons as originally constructed had submerged, 
partial-mix aeration tubing in the first and second aeration 
cells, and there were no baffles in place. In effect, the first 
baffle in the first lagoon cell converts the entry zone into a 
complete-mix aeration component. The purpose of these 
modifications was to obtain a more rapid removal of BOD 5 
and more effective settling of TSS in the lagoons, and to 
subsequently permit more effective ammonia removal in 


Table 8-10. Water Quality Performance, Mandeville LA Treatment 
System, June/September 1991 


Parameter 

System 

Influent 

Wetland 

Influent 

Wetland 

Effluent 

BOD (Total) mg/L 

154 

41 

10 

BOD (Soluble) mg/L 

ND' 

21 

8 

COD (Total) mg/L 

349 

79 

43 

COD (Soluble) mg/L 

ND 

40 

31 

TSS mg/L 

132 

59 

7 

VSS mg/L 

ND 

39 

5 

TKN mg/L 

ND 

5 

3 

NH 4 -N mg/L 

ND 

1.4 

2.1 

Organic N mg/L 

ND 

3.1 

1.1 

N0 3 -N mg/L 

ND 

4 

0.2 

TN mg/L 

ND 

9 

3 

TP mg/L 

ND 

3 

4 

Fecal Coliforms#/100ml 

ND 

TNTC 

TNTC 2 

DO mg/L 

ND 

2.4 

1.8 

pH 

6.9 

6.9 

7.0 

Temperature °C 

ND 

31.8 

30.5 


'ND = No data available. 

2 TNTC = Too numerous to count, sample taken prior to disinfection 


the lagoons and VSB component. This strategy has been 
successful for BOD 5 and TSS, but not for ammonia. The 
low ammonia values obtained in the EPA study during the 
1991 summer are misleading. The records for the entire 
year show a seasonal trend in effluent concentrations that 
are similar to those shown in Table 8-11 for 1996-1997. In 
1991, the effluent ammonia concentration averaged 3.2 
mg/L during the warm months (March-November) and 7.8 
mg/L during the colder months (December-February). The 
system met the ammonia limit by a significant margin dur¬ 
ing that first year of operation. 

There are no significant seasonal trends in the ammo¬ 
nia concentration in the untreated wastewater, but there 
are in the lagoon effluent, which indicates that these higher 
winter values are not treated effectively by the VSB. As a 
result, the system effluent exceeds the discharge limit. This 
condition suggests that the lagoon, as presently config¬ 
ured, does not provide effective nitrification during the 
colder weather. That is a plausible hypothesis since the 
nitrifier organisms are temperature sensitive and gener¬ 
ally exist in relatively low numbers in these partial-mix aer¬ 
ated lagoons with no sludge return. 

The city intends to increase the capacity of the system 
to about 4 mgd to keep up with expected growth in the 
community. Discussions are underway regarding the fu¬ 
ture system configuration to solve both the ammonia prob¬ 
lem and permit capacity expansion with maximum utiliza¬ 
tion of the existing facilities. 

8.2.3.5 Lessons Learned 

• The internal hydraulics of the wetland cells force all of 
the influent to enter the cell at three points, with a total 
cross-sectional area of about 2 ft 2 , which is inadequate 
to receive a design flow of about 350 gpm and results 
in surface flow in the inlet zone. A perforated inlet 
manifold that extended the full width of each cell should 
have prevented surface flow. At the effluent end of the 
cells, surface flow was caused by outlet ports installed 
at the same elevation as the rock surface. 

• A means of controlling water levels in the bed and al¬ 
lowing the bed to be drained for maintenance would 
have improved the system. 

• Modifications to the system, including additional ori¬ 
fices drilled in the effluent manifold in the side and lower 
quadrant, additional surface gravel placed in the area 
of the manifold, and a new pipe installed to permit drain¬ 
age of the cells, resulted in a lowering of the water 
level in the effluent zone of the wetland bed, so the 
gravel surface in that area is generally dry. 

• Surface flow was experienced almost immediately in 
the inlet and outlet zones of this system and was not 
caused by clogging, as confirmed by EPA investiga¬ 
tions in 1991, but rather by lack of hydraulic gradient. 

• Hydraulic gradient for a flat-bottomed system can be 
provided with a water-level control device at the efflu¬ 
ent end of the cell. 


149 













Table 8-11. Water Quality Performance, Mandeville, LA, Treatment System, 1996 -1997 


Date 

Avg. Flow 
mgd 

BOD 

mg/L 

Raw Wastewater 
TSS 
mg/L 

nh 4 -n 

mg/L 

BOD 

mg/L 

Wetland Influent 
TSS 
mg/L 

nh 4 -n 

mg/L 

BOD 

mg/L 

Wetland Effluent 
TSS 
mg/L 

nh 4 -n 

mg/L 

1996 

Jan 

1.57 

133 

115 

14 

15 

8 

14.3 

5 

2 

11.9 

Feb 

1.60 

156 

120 

15 

14 

8 

14 

6 

2 

12 

Mar 

1.75 

126 

116 

13 

16 

11 

16 

2 

2 

11 

Apr 

1.26 

145 

148 

14 

11 

8 

12 

4 

1 

8 

May 

1.11 

137 

133 

18 

45 

25 

10 

3 

2 

9 

Jun 

1.72 

138 

132 

18 

27 

18 

1 

2 

2 

0.5 

Jul 

1.33 

131 

115 

16 

24 

24 

1 

2 

1 

0.7 

Aug 

1.71 

64 

70 

20 

30 

48 

7 

10 

7 

5 

Sep 

1.32 

61 

155 

31 

59 

20 

12 

9 

7 

5 

Oct 

1.69 

86 

137 

21 

43 

18 

14 

7 

4 

5 

Nov 

1.51 

119 

126 

44 

62 

8 

8 

8 

5 

4 

Dec 

1.57 

116 

122 

37 

43 

11 

28 

6 

2 

19 

Avg 

1.51 

118 

124 

22 

32 

17 

11 

5 

3 

8 

1997 

Jan 

1.85 

89 

111 

20 

70 

14 

12 

7 

4 

7 

Feb 

2.31 

63 

94 

24 

62 

23 

44 

10 

6 

20 

Mar 

1.53 

94 

128 

26 

59 

16 

16 

7 

3 

5 

Apr 

1.44 

97 

98 

22 

52 

20 

10 

6 

4 

4 

May 

1.67 

112 

140 

25 

34 

14 

6 

4 

3 

4 

Jun 

1.64 

93 

167 

28 

57 

10 

12 

4 

3 

4 

Jul 

1.68 

108 

109 

16 

51 

9 

9 

6 

2 

5 

Avg 

1.73 

94 

121 

23 

55 

15 

16 

6 

4 

7 

Weighted 

1.59 

109 

123 

22 

40 

16 

13 

5 

3 

8 

Average 

1996/97 

1991 

1.16 

154 

132 

ND 

41 

59 

1.4 

10 

7 

2.1 


• Bulrush planted in this system has attracted nutria and 
muskrat, which favor bulrush for food and nesting 
material. Nutria have eaten most of the bulrush plants 
and bored through the interior berms, which causes 
significant leakage between the cells. Small sacks filled 
with a mixture of cement and sand have corrected the 
leakage problem of nutria boring through interior 
berms. 

• The minimal availability of oxygen in VSB wetland beds 
makes them ineffective for nitrification of ammonia, and 
the Mandeville system can meet ammonia discharge 
limits only when the aerated lagoon provides sufficient 
ammonia removal. 

8.2.4 Sorrento, Louisiana 

8.2.4.1 Background 

Sorrento, a small residential community in southeastern 
Louisiana, is located about 50 miles southeast of Baton 
Rouge. Prior to construction of the aerated lagoon wet¬ 
land system, the community was served by on-site septic 
tank systems. Many of these on-site systems were not func¬ 
tioning properly due to the difficult soil conditions in the 
area. The new system was designed in 1990 and placed 
in operation in late 1991. The lagoon component consists 
of two 10-ft-deep aerated cells (first cell contains four 3-hp 
floating aerators, second cell contains four 2-hp floating 


aerators), followed by a 7-ft-deep settling pond. The two 
aerated cells were designed for 10 d HRT at the potential 
ultimate flow rate of 130,000 gpd. At the 1997 flow rate of 
32,000 gpd, the HRT is about 40 d, and only a few of the 
aerators were operated. 

The VSB cell, with a bottom area of about 7800 ft 2 , was 
designed for a flow rate of 50,000 gpd with the intention of 
adding a second parallel cell as the flow rate increases in 
the future. The design HRT in this bed at 50,000 gpd would 
be one day. The native soils are clays and silty clays, so 
the bottoms of the lagoon cells and the VSB cells are not 
lined. However, a geotextile liner is used on the inner slope 
of all berms to prevent erosion and weed growth; the outer 
slope of these berms is grassed. The system discharges 
to Bayou Conway, and the NPDES discharge limits are 
BOD 5 20 mg/L, TSS 20 mg/L, pH 6-9, and fecal coliforms 
200-400/100 mL. There are no ammonia limits for this sys¬ 
tem. 

As shown in Figure 8-9, the VSB cell is triangular in 
shape, with the inlet zone about 60 ft wide and the flow 
path to the outlet about 250 ft long. This shape was se¬ 
lected to minimize short-circuiting of flow. Previous designs 
had large aspect ratios (10:1 or greater) but insufficient 
hydraulic gradient to overcome the frictional resistance, 
resulting in surface flow on top of the bed. At Sorrento, the 
average aspect ratio is only 6:1, but all of the flow con¬ 
verges at the end of the triangular bed. 


150 









Aerators 


Influent 


Discharge Chlorination 



Aerated Lagoons 


Vegetated Bed 
60 X 250 ft 


Settling Pond 


Figure 8-9. Schematic of VSB system at Sorrento, LA 


The design engineer of the Sorrento system also incor¬ 
porated several features to ensure that an adequate hy¬ 
draulic gradient would always be available based on les¬ 
sons learned from other systems. The bottom of the bed is 
flat and level throughout its length, but the gravel depth is 
3 ft at the inlet and 2.5 ft at the outlet, so the top surface of 
the gravel slopes to provide for 0.5 ft of headloss. In addi¬ 
tion, the single outlet structure contains an adjustable sluice 
gate that allows a further increase in the available hydrau¬ 
lic gradient by adjusting the water level in the bed. The 
inlet to the bed is an 8-in perforated pipe resting on the 
bottom of the bed and extending the full width. The bed 
effluent discharges to a concrete outlet box. There is also 
an 8-in valved drain pipe at the outlet end of the bed to 
drain the cell completely, if necessary. A chlorine contact 
chamber is provided for disinfection prior to final discharge. 

Two layers of aggregate are used in the Sorrento VSB. 
The top layer is a 6-in depth of washed, 0.75-in gravel. 
The main part of the bed is composed of crushed lime¬ 
stone imported from Mexico, ranging from 1.5 to 3 in in 
size. Since ammonia removal was not required and be¬ 
cause maintenance problems with vegetation were appar¬ 
ent at other systems, it was decided not to plant vegeta¬ 
tion on the Sorrento VSB cell. At the time of the 1997 in¬ 
spection for this report, weeds were growing around the 
fringes of the wetland bed, but the general bed surface 
was still free of vegetation. 

This wetland system was selected for use at Sorrento 
because the facility planning evaluation showed it to be 
the most cost-effective process for meeting the NPDES 
discharge requirements. The total construction cost for the 


entire system was about $233,400 (1991$), with an esti¬ 
mated $75,000 for the VSB component. The unit construc¬ 
tion cost for the VSB would then be about $1500 per 1000 
gallons of design capacity (for the 50,000 gpd design flow). 
On an area basis, the capital costs would be about 
$419,000 per acre for the 0.18-acre VSB. 

8.2.4.2 Financial Arrangements 

The construction costs for the Sorrento system were 
funded with federal and state money provided under the 
U.S. EPA Construction Grant Program that existed at that 
time. The O&M costs for the system are supported by sewer 
fees from the connected users. 

8.2.4.3 Construction and Start-up Procedures 

A site for the new lagoon/wetland system was identified 
on available land between the community and the final dis¬ 
charge point to Bayou Conway. Geotechnical investiga¬ 
tions were undertaken to identify and characterize the in 
situ soils. These proved to be clays and silty clays that 
would provide adequate protection for ground water. There 
also was no identified risk of ground water intrusion or sur¬ 
face water flooding at this site. 

The site is relatively level, so the entire system was ex¬ 
cavated, with excess material used to construct the berms. 
A 3-ft freeboard was provided for the lagoon cells and the 
VSB component. The rock and gravel were hauled by truck 
from a barge dock on the Mississippi River, dumped into 
the bed, and spread with a small bulldozer. The entire 
coarse rock layer was placed and leveled before any gravel 
was placed as the top layer. Inlet and outlet manifolds were 
then installed and connected and rock backfilled around 


151 





















them. The bed was then filled with effluent from the lagoon 
to the top of the coarse rock. Since vegetation was not 
used on this system, start-up was immediate. 

Routine maintenance procedures include servicing of the 
lagoon aerators and the chlorine disinfection equipment; 
there are no routine maintenance requirements for the 
wetland bed. There have been problems with nutria bur¬ 
rowing in the banks of the lagoon cells; however, since 
there is no vegetation and no exposed water in the wet¬ 
land cell, these animals have not been a problem at this 
site. Since maintenance has not been required for the 
wetland cell, the O&M cost for this component is zero. 

8.2.4.4 Performance History 

Water quality data are not available for untreated sew¬ 
age at Sorrento or for lagoon effluent entering the wetland 
system. The 1997 flow is estimated to be in the range of 
15,000 gpd. At that rate the HRT would be about 100 d in 
the lagoons and 4 d in the VSB component. With such a 
long HRT, lagoon effluent could be expected to have a 
BOD b of less than 20 mg/L and a TSS in the same range 
(except for algal bloom periods). With inputs at this level, 
the VSB with an HRT of 4 d could be expected to produce 
background levels of BOD 5 and TSS, as confirmed in Table 
8 - 12 . 

8.2.4.5 Lessons Learned 

• The triangular configuration of this VSB system causes 
flow lines to converge at the end of the system in a 
single outlet point, which is cost effective, but any such 
design would need to evaluate weir loading rates per 
unit length to avoid excessive velocity in the outlet zone 
that could cause resuspension of TSS and its associ¬ 
ated contaminants. 


• The sloping surface of the gravel (0.2% grade) pro¬ 
vides an additional 0.5 ft of potential head at the inlet 

Table 8-12. VSB Effluent Water Quality, Sorrento, LA 


Date 

BOD, mg/L 

TSS, mg/L 

Fecal Coli' 

pH 

2/23/94 

<6 

5 

0 

7.2 

3/31/94 

<6 

4 

0 

7.2 

5/18/94 

<6 

2 

20 

7.5 

7/28/94 

<6 

4 

100 

7.4 

8/10/94 

<6 

4 

0 

7.2 

9/26/94 

<6 

4 

6 

7.8 

12/30/94 

<6 

4 

180 

7.6 

1/12/95 

<6 

4 

TNTC 

7.9 

3/8/95 

<6 

4 

4200 

7.9 

4/28/95 

<6 

4 

666 

7.1 

6/16/95 

<6 

11 

0 

7.8 

7/12/95 

<6 

4 

0 

7.5 

8/10/95 

<6 

4 

0 

7.3 

9/13/95 

<6 

4 

0 

7.5 

10/11/95 

<6 

4 

0 

7.4 

11/8/95 

<6 

4 

0 

7.5 

12/13/95 

<6 

4 

53 

7.4 

1/24/96 

<6 

4 

350 

7.4 

2/14/96 

<6 

4 

3 

7.5 

3/13/96 

<6 

4 

6 

7.0 


'Note: after chlorine disinfection, #/100 ml 


to help ensure that the hydraulic gradient is sufficient 
to avoid surface flow on the bed. 

• The adjustable outlet gate provides additional water 
level adjustment; however, the outlet gate cannot be 
lowered completely, so an additional drain pipe for de¬ 
watering the bed is necessary. A completely adjust¬ 
able outlet may have eliminated both the additional 
gravel necessary to produce the sloping surface and 
the drain pipe for dewatering. 

• The system is oversized for the current flow rate and 
organic loading, so an additional VSB cell may not be 
necessary for the system to handle flow rate increases 
anticipated in the future. 

• The lack of plants in this system does not appear to 
affect removal of BOD and TSS, as was observed in a 
1992 EPA performance evaluation of a vegetated sys¬ 
tem and a temporarily nonvegetated system. 

• The lack of plants in effect equates the VSB concept 
to a horizontal-flow, coarse-media, contact filter. 

8.3 Lessons Learned 
8.3.1 Design 

Organic Loading 

Organic loadings in the range of 10 to 25 lbs BOD/acre/ 
day to FWS systems have been shown to effectively meet 
30 mg/L BOD and TSS monthly effluent standards, with 
no need after 15 years of operation to remove the settled 
material from a FWS system. For the majority of these 
systems, this range of organic loading results in six to eight 
days of theoretical hydraulic retention. 

D ata Gaps 

The database for both VSB and FWS systems has a 
continuing problem of not having enough quality-assured 
data to evaluate removal rates, seasonal differences, wa¬ 
ter balances, and long-term treatment effectiveness. In¬ 
sufficient data have been collected on contaminant load¬ 
ings (both flow and concentration), incremental data 
through the system (multiple data collection points), and 
water column data (temperature, pH, and dissolved oxy¬ 
gen) at different locations. 

Inlet/Outlet Works 

Studies comparing the placement of inlet/outlet (I/O) 
works as a function of cell geometry and outlet approach 
conditions are lacking. As a result, there is not a complete 
rational approach to the placement and design of inlet and 
outlet works for these types of systems. For example, the 
criterion of weir overflow rate typically has been used to 
place outlet weirs and specify weir length in these low ap¬ 
proach-velocity systems. Included in the outlet design are 
bathymetric and vegetative conditions of the outlet zone 
of FWS wetlands. Large collection areas immediately up¬ 
stream from outlet works that have no emergent vegeta- 


152 








tion have resulted in poor effluent quality. Relatively shal¬ 
low collection zones with emergent vegetation have shown 
less variability of effluent quality, but more O/M require¬ 
ments. 

Headloss 

While headloss is a factor of concern in VSB systems, it 
is not a major factor in FWS wetlands unless they have 
extremely long and narrow flow reaches. Headloss is a 
consideration in FWS wetlands only when L:W ratios are 
great (10:1 or greater) and are combined with high hy¬ 
draulic loading rates in heavily vegetated cells. Proper 
placement of I/O works, sufficient berm height, and L:W 
ratios less than 10:1 minimize headloss effects in FWS 
wetlands. Clean-water headloss through a VSB wetland 
system is quickly modified as pore spaces are filled with 
separated solids and, to a lesser degree, rhizosphere de¬ 
velopment. Under conditions of plugging by these mecha¬ 
nisms, the liquid level will eventually surface and an un¬ 
dersized, fully vegetated FWS wetland condition will be¬ 
gin to develop on the surface. 

Hy d r aulics 

The internal hydraulics of a VSB system are critical to 
treatment success. Systems constructed with high L:W 
ratios, flat bottoms, and with effluent manifold ports lo¬ 
cated at the top of the gravel produced surface flow at the 
effluent ends of the cell. These types of systems also did 
not allow for water level control within the bed and cell 
drainage. Multiple-ported influent manifolds extending the 
width of the inlet zone, coupled with similar effluent col¬ 
lection manifolds with adjustable weirs or rotating elbows, 
would allow for greater hydraulic contact in the basin and 
more operational flexibility. Sloping the surface of the 
gravel bed based on the design flow headloss may also 
permit increases in VSB hydraulic loading and duration of 
service prior to major inlet maintenance. 

Aspect Ratio 

High aspect ratio FWS wetland cells can produce sig¬ 
nificant operational problems at high hydraulic loading 
rates and/or with dense stands of emergent vegetation. 
Headloss effects are additive and are greatly aggravated 
with high length-to-width ratios in totally vegetated FWS 
wetlands. Both types of systems with L:W ratios as high 
as 30:1 have produced significant flooding at the influent 
berms while dropping the effluent water elevation below 
weir recovery levels. Parallel cells with lower L:W with 
multiple I/O works can control this effect. The only design 
limitation is that the HRT must be above some minimum 
to assure removal of TSS and associated pollutants of 
concern. 

Ice Formation 

In colder climates where ice forms on standing water, 
sufficient freeboard and outlet control is essential to allow 
for ice formation to cap the normal operating depth of the 


free surface water column. In most cases, this distance is 
less than 1 ft. The ability to operate the wetland with the 
water column directly in contact with the ice, with no low¬ 
ering of the water level once the ice forms, is another im¬ 
portant design feature. Lowering the water under these 
conditions could allow for a secondary ice level, with a 
liquid level constraint, to form under the primary level. 

8.3.2 Mechanisms and Processes 

Oxvoen Transfer 

Oxygen transfer through the rhizosphere evidently is not 
a major contributor to contaminant oxidation in vegetated 
submerged bed systems, based on both research and full- 
scale studies. Oxygen demand associated with storage 
products in roots and tubers is much greater than excess 
oxygen available at the root hairs and other plant parts. In 
FWS wetlands, epiphytes can colonize on stems and 
leaves preferentially depending on oxygen exuding from 
the gas transport plant structures. 

Nitrification-Denitrification in VSB Systems 

Cost-effectively sized VSB systems have not been shown 
to significantly nitrify treated influent. It follows that VSBs 
have not been shown to be able to denitrify an influent, 
which is predominantly ammonia. Early studies that sug¬ 
gested that significant amounts of nitrogen can be removed 
in a VSB wetland system have not been duplicated in sub¬ 
sequent studies and full-scale evaluations. 

Plant Coverage 

Coverage by emergent plants in FWS wetlands should 
not be 100% because too much coverage by emergent 
plants is negatively correlated with effluent quality. Plac¬ 
ing open water (submergent aquatic macrophytes) between 
areas of closed water (emergent macrophytes) is corre¬ 
lated with better effluent quality than a wetland that has 
100% emergence. Submergent plants release oxygen to 
the water column, and these open water zones allow for 
more surface reaeration. Emergent plants also contribute 
more internal BOD loading upon decomposition. 

Ground Water Recharge 

Siting and designing FWS wetlands with intentional dis¬ 
charge to ground water is a legitimate application of this 
treatment technology. With proper design, FWS wetlands 
for ground water recharge can remove nitrate nitrogen 
(through denitrification) and indicator organisms. Data col¬ 
lected at “leaky wetland sites,” such as in Jackson Bot¬ 
tom, Oregon, have demonstrated the effectiveness of these 
processes in locations where soils are sufficiently porous. 

Plant Litter 

Plant litter is an essential component of a FWS con¬ 
structed wetland. While this material contributes to flow 
resistance, it more importantly seals surface areas in fully 
vegetated zones to assure anoxic conditions. 


153 













8.3.3 Vegetation 

Development of Treatment Effectiveness 

The full treatment potential of a FWS system may not be 
realized until there is both full coverage of plants (as de¬ 
signed) and a layer of litter beginning to accumulate on 
the surface of the water. Depending on the type of plants, 
planting density, and time of planting, a minimum of two 
growing seasons, or two to three years, may be needed. 
This may be important when negotiating discharge permit 
requirements. 

Ammonia Nitrogen 

In VSB systems, plants are not critical to the removal of 
BOD and TSS, as shown in several systems. Evidence 
suggests, however, that they are effective in removing 
portions of the nitrogen and phosphorus due to uptake by 
the plants during the growing season. Most of these nutri¬ 
ents are returned to the water column during the senes¬ 
cent period. 

Aeration 

Attempts to aerate outlet zones of FWS systems with 
submerged tubing have resulted in attracting animals such 
as muskrats and nutria, which may damage the tubing. 

Short-circuiting 

Vegetation predation by nutria and muskrats in FWS 
wetlands can produce serious hydraulic short-circuiting. 
Varying plant resistance as the wastewater moves through 
the wetland also can cause short-circuiting. Preferential 
flow routes can develop in these systems, which have rela¬ 
tively low velocities. 

Seeding and Germination 

Hydroseeding both VSB and FWS wetlands has only 
been successful for cattails in some instances. Additional 
studies of this low-cost approach to planting are recom¬ 
mended. Seeding with 25 Ib/acre mixed with a mulch at 
2500 Ib/acre have been used for VSB systems. Continu¬ 
ous shallow-water inundation has consistently produced 
higher germination rates than have sprinklers. 

Plant Toxicity 

Emergent vegetation species, such as cattails and bul¬ 
rushes, are sensitive to deep anaerobic sludge banks. In 
situations with large volumes of carryover solids and mal¬ 
functioning activated sludge units, emergent vegetation in 
the inlet zone can die from sulfide toxicity in the rhizosphere. 
Wastewaters with high levels of suspended and settle- 


able solids should be pretreated upstream from a wetland 
system through use of a settling pond. 

8.3.4 Treatment Effectiveness 

Nutrient Uptake 

Both FWS and VSB wetlands have been shown to be 
unable to reduce levels of BOD, dissolved phosphorus, 
and ammonia nitrogen below certain minimums. This is 
due to internal processes in a wetland, such as the solubi¬ 
lization of influent settleable/suspended solids and the lit¬ 
ter layer of aquatic macrophytes. Depending on the cli¬ 
mate, pulses of dissolved carbon (both degradable and 
non-degradable), soluble reactive phosphorus, and am¬ 
monia nitrogen are taken up by the plants, and they are 
released during periods of active decomposition in the 
wetland. Colder climatic conditions with early falls, long 
cold winters, and warm springs will pulse these materials 
into the water column during the spring warm-up period. 

Nitrification 

Without operating in a fill-and-draw batch mode, it is not 
economically feasible to attain aerobic conditions in a VSB 
system to convert ammonia to nitrate. A VSB is anaerobic 
throughout most of its depth, with little opportunity for nitri¬ 
fying bacteria populations to develop. Internally loaded 
ammonia from the decomposition of algal cells has also 
been shown to be a factor when attempting to use a VSB 
to meet ammonia standards. 

Sheet Flow for Nitrification 

Attempts to operate a fully vegetated FWS wetland in a 
shallow mode to simulate conditions of overland flow have 
not proven to be effective. 

Measurement of Treatment Effectiveness 

Most of the FWS wetlands in the NADB are used to treat 
high-quality influents producing low organic loading con¬ 
ditions. In most of these cases, the internal load is more 
significant than the influent load. In only a few cases with 
high organic loading rates were the upper limits of treat¬ 
ment effectiveness measured, such as Gustine, Califor¬ 
nia. While many viewed Gustine as a failure, it provided 
well-documented data on a wide range of BOD, TSS, ni¬ 
trogen, and coliform fully vegetated zone loading condi¬ 
tions. These data showed that the upper instantaneous 
BOD loadings of 150 to 200 Ibs/acre/day could still result 
in less than 40 mg/L BOD in the effluent. Such loading 
rates were based on a fully vegetated FWS system and a 
specific wastewater, so the utility of this information is lim¬ 
ited. For example, if a wastewater had a soluble BOD load¬ 
ing of this magnitude, it would not be prudent (or success¬ 
ful) to use a single fully vegetated cell for treatment. 


154 
















































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